null pointer exception and not able show data in listview

Question

I want to retrieve contact data from Firebase Realtime Database and show them in a ListView in Android studio. Problem is my code is getting an Error of null pointer exception. I ran the debugger before this exception it was running fine but suddenly I got this error.

I might also add that after retrieving data from firebase I kept them in an ArrayList. when I ran the debugger the ArrayList was showing its size according to the database but when it got to the adapter class it became null. The ListView that I have created to show contacts is empty. It does not show any data even after notifying the adapter that the data have changed.

Can anyone help me with this?

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.sosandvideo, PID: 24758
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sosandvideo/com.example.sosandvideo.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:389)
        at android.widget.ListView.setAdapter(ListView.java:581)
        at com.example.sosandvideo.MainActivity.onCreate(MainActivity.java:43)
        at android.app.Activity.performCreate(Activity.java:7994)
        at android.app.Activity.performCreate(Activity.java:7978)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

 This is my MainActivity.class

package com.example.sosandvideo;

import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.content.Intent;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.provider.ContactsContract;
import android.view.View;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;

;

import com.example.sosandvideo.Contacts.Contact;
import com.example.sosandvideo.Contacts.ContactAdapter;
import com.example.sosandvideo.Contacts.DBhelper;

import java.util.List;

public class MainActivity extends AppCompatActivity {
    private static final int RESULT_PICK_CONTACT =1;
    ListView listview;
    List<Contact> list;
    ContactAdapter contactAdapter;
    private Button add;
    DBhelper db=new DBhelper();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate (savedInstanceState);
        setContentView (R.layout.activity_main);


       // phone = findViewById (R.id.contactTv);
        add = findViewById (R.id.addButton);
        listview=(ListView)findViewById(R.id.listView);
        list = db.getAllContacts();
        contactAdapter=new ContactAdapter(this,list);
        listview.setAdapter(contactAdapter);

        add.setOnClickListener (new View.OnClickListener () {
            @Override
            public void onClick(View v) {
                Intent in = new Intent (Intent.ACTION_PICK, ContactsContract.CommonDataKinds.Phone.CONTENT_URI);
                startActivityForResult (in, RESULT_PICK_CONTACT);
            }
        });

    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode,  Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == RESULT_OK) {
            switch (requestCode) {
                case RESULT_PICK_CONTACT:
                    contactPicked(data);
                    break;
            }
        } else {
            Toast.makeText(this, "Failed To pick contact", Toast.LENGTH_SHORT).show();
        }
    }

    @SuppressLint("Range")
    private void contactPicked(Intent data) {
        Cursor cursor = null;

        try {
            String phoneNo = null;
            int id;
            String name;
            Uri uri = data.getData ();
            cursor = getContentResolver ().query (uri, null, null,null,null);
            cursor.moveToFirst ();
             id= cursor.getColumnIndex (ContactsContract.CommonDataKinds.Phone.CONTACT_ID);

            String phoneid= cursor.getString (id);
            String phoneNum= cursor.getString(cursor.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER));
            name=cursor.getString(cursor.getColumnIndex(ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME));
            db.addContact(new Contact(phoneid,phoneNum,name));
            list = db.getAllContacts();
            contactAdapter.refresh(list);

        } catch (Exception e) {
            e.printStackTrace ();
        }
    }
}

This is My DBhelper.class

package com.example.sosandvideo.Contacts;


import android.content.Context;
import android.util.Log;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import org.jetbrains.annotations.NotNull;

import java.util.ArrayList;
import java.util.List;

public class DBhelper {
    FirebaseDatabase database=FirebaseDatabase.getInstance();
    DatabaseReference reference= database.getReference("contacts");
    ArrayList<Contact> list;

    public void addContact(Contact contact)
    {
//        added contact details to database

        reference.child(contact.getId()).setValue(contact);

    }
    public void deleteContact(Contact contact)
    {

        reference.child(contact.getId()).removeValue();
    }
    public List<Contact> getAllContacts()
    {

       reference.addChildEventListener(new ChildEventListener() {
           @Override
           public void onChildAdded(@NonNull @NotNull DataSnapshot snapshot, @Nullable @org.jetbrains.annotations.Nullable String previousChildName) {
               Contact contact=snapshot.getValue(Contact.class);
               list.add(contact);
               //reference.setValue(list);

           }

           @Override
           public void onChildChanged(@NonNull @NotNull DataSnapshot snapshot, @Nullable @org.jetbrains.annotations.Nullable String previousChildName) {

           }

           @Override
           public void onChildRemoved(@NonNull @NotNull DataSnapshot snapshot) {

           }

           @Override
           public void onChildMoved(@NonNull @NotNull DataSnapshot snapshot, @Nullable @org.jetbrains.annotations.Nullable String previousChildName) {

           }

           @Override
           public void onCancelled(@NonNull @NotNull DatabaseError error) {

           }
       });
       return list;

    }


}

This is my Adapter class

package com.example.sosandvideo.Contacts;

import android.content.Context;
import android.content.DialogInterface;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;

import com.example.sosandvideo.R;
import com.google.android.material.dialog.MaterialAlertDialogBuilder;

import java.util.List;

public class ContactAdapter extends ArrayAdapter<Contact> {

    List<Contact> list;
    Context context;

    public ContactAdapter( @NonNull Context context ,List<Contact> list)
    {
        super(context,R.layout.item,list);
        this.context=context;
        this.list=list;
    }
    @Override
    public View getView(int position, View convertView, ViewGroup parent)
    {
        DBhelper db=new DBhelper();
        Contact c = getItem(position);

        // Check if an existing view is being reused, otherwise inflate the view
        if (convertView == null) {
            convertView = LayoutInflater.from(getContext()).inflate(R.layout.item, parent, false);
        }

        LinearLayout linearLayout = (LinearLayout) convertView.findViewById(R.id.linear);

        // Lookup view for data population
        TextView tvName = (TextView) convertView.findViewById(R.id.tvName);
        TextView tvPhone = (TextView) convertView.findViewById(R.id.tvPhone);

        // Populate the data into the template
        // view using the data object
        tvName.setText(c.getName());
        tvPhone.setText(c.getPhoneNum());

        linearLayout.setOnLongClickListener(new View.OnLongClickListener() {
            @Override
            public boolean onLongClick(View view) {
                // generate an MaterialAlertDialog Box
                new MaterialAlertDialogBuilder(context)
                        .setTitle("Remove Contact")
                        .setMessage("Are you sure want to remove this contact?")
                        .setPositiveButton("YES", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialogInterface, int i) {
                                // delete the specified contact from the database
                                db.deleteContact(c);
                                // remove the item from the list
                                list.remove(c);
                                // notify the listview that dataset has been changed
                                notifyDataSetChanged();
                                Toast.makeText(context, "Contact removed!", Toast.LENGTH_SHORT).show();
                            }
                        })
                        .setNegativeButton("NO", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialogInterface, int i) {

                            }
                        })
                        .show();
                return false;
            }
        });
        // Return the completed view to render on screen
        return convertView;
    }

    // this method will update the ListView
    public void refresh(List<Contact> c_list) {
        list.clear();
        list.addAll(c_list);
        notifyDataSetChanged();
    }
}