Attempt to invoke virtual method on a null object reference(Android Studio app stopped)

Question

I tried to develop a browser app on android studio, but when I open it, I see a message which says:"App unfortunately stopped!" .I see Logcat and I see an error:

03-23 17:00:08.021 2155-2155/com.pliamdev.pliam.pliambrowser E/AndroidRuntime: FATAL EXCEPTION: main Process: com.pliamdev.pliam.pliambrowser, PID: 2155

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.pliamdev.pliam.pliambrowser/com.pliamdev.pliam.pliambrowser.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3132) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415) at android.app.ActivityThread.access$1100(ActivityThread.java:229) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:7325) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.support.v7.app.AppCompatDelegateImplBase.(AppCompatDelegateImplBase.java:117) at android.support.v7.app.AppCompatDelegateImplV9.(AppCompatDelegateImplV9.java:149) at android.support.v7.app.AppCompatDelegateImplV11.(AppCompatDelegateImplV11.java:29) at android.support.v7.app.AppCompatDelegateImplV14.(AppCompatDelegateImplV14.java:54) at android.support.v7.app.AppCompatDelegateImplV23.(AppCompatDelegateImplV23.java:31) at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:200) at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:183) at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519) at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190) at com.pliamdev.pliam.pliambrowser.MainActivity.(MainActivity.java:12) at java.lang.Class.newInstance(Native Method) at android.app.Instrumentation.newActivity(Instrumentation.java:1096) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3122) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415) at android.app.ActivityThread.access$1100(ActivityThread.java:229) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:7325) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

Here is my main activity:

    package com.pliamdev.pliam.pliambrowser;

    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.webkit.WebView;
    import android.widget.Button;
    import android.widget.EditText;


    public class MainActivity extends AppCompatActivity {
    Button button=(Button) findViewById(R.id.button);
    EditText editText=(EditText) findViewById(R.id.editText);
    WebView webView=(WebView) findViewById(R.id.webView);
    @Override
        protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        webView.loadUrl("https://www.google.gr");
        button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            webView.loadUrl(editText.getText().toString());
        }
    });
    }
    }

and here is my main xml file:

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.constraint.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.pliamdev.pliam.pliambrowser.MainActivity">

    <WebView
    android:id="@+id/webView"
    android:layout_width="match_parent"
    android:layout_height="426dp"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

    <Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginTop="8dp"
    android:background="@color/colorPrimaryDark"
    android:text="@string/go_url"
    app:layout_constraintBottom_toTopOf="@+id/webView"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

    <EditText
    android:id="@+id/editText"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:ems="10"
    android:hint="@string/enter_url"
    android:inputType="textPersonName"
    android:labelFor="@+id/editText"
    android:text=""
    app:layout_constraintBottom_toTopOf="@+id/webView"
    app:layout_constraintEnd_toStartOf="@+id/button"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />
    </android.support.constraint.ConstraintLayout>

What can I do in order for my application to work?