Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object

Question

context

I could not able to retrive data from firebase. While retriving the data getting bellow error. Kindly help

2020-12-11 14:21:19.954 8628-8628/com.troll.trollmemes E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.troll.trollmemes, PID: 8628
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
        at com.troll.trollmemes.Fragment.HomeFragment$2.onDataChange(HomeFragment.java:95)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@16.0.4:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@16.0.4:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@16.0.4:55)
        at android.os.Handler.handleCallback(Handler.java:790)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

below is my code

public class HomeFragment extends Fragment {

    private RecyclerView recyclerView;
    private PostAdapter postAdapter;
    private List<Post>postList;

    private List<String> followingList;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_home, container, false);

        recyclerView = view.findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        LinearLayoutManager mLayoutManager = new LinearLayoutManager(getContext());
        mLayoutManager.setReverseLayout(true);
        mLayoutManager.setStackFromEnd(true);
        recyclerView.setLayoutManager(mLayoutManager);
        postList = new ArrayList<>();
        postAdapter = new PostAdapter(getContext(),postList);
        recyclerView.setAdapter(postAdapter);

        checkFollowing();

        return view;
    }

    private void checkFollowing(){
        followingList = new ArrayList<>();


        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Follow")
                .child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                .child("following");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                followingList.clear();
                for (@NonNull DataSnapshot snapshot: dataSnapshot.getChildren()){

                    followingList.add(snapshot.getKey());
                }
                readPosts();
                //new
                //postAdapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    private void readPosts(){
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Posts");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                postList.clear();
                for (@NonNull DataSnapshot snapshot : dataSnapshot.getChildren()){
                    Post post = snapshot.getValue(Post.class);
                    for (String id : followingList){
                        if (post.getPublisher().equals(id)){
                            postList.add(post);
                        }
                        postAdapter.notifyDataSetChanged();
                    }
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }
}

in readPosts i am getting the issue

public class PostAdapter extends RecyclerView.Adapter<PostAdapter.ViewHolder>{

    public Context mContext;
    public List<Post> mPost;

    private FirebaseUser firebaseUser;

    public PostAdapter(Context mContext, List<Post> mPost)
    {
        this.mContext = mContext;
        this.mPost = mPost;
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
        View view = LayoutInflater.from(mContext).inflate(R.layout.post_item,viewGroup, false);

        return new PostAdapter.ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder viewHolder, int i) {

        firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        Post post = mPost.get(i);

        Glide.with(mContext).load(post.getPostimage()).into(viewHolder.post_image);

        if (post.getDescription().equals("")){
            viewHolder.description.setVisibility(View.GONE);
        } else {
            viewHolder.description.setVisibility(View.VISIBLE);
            viewHolder.description.setText(post.getDescription());
        }

        publisherInfo(viewHolder.image_profile, viewHolder.username, viewHolder.publisher, post.getPublisher());

    }

    @Override
    public int getItemCount() {
        return mPost.size();
    }


    public class ViewHolder extends RecyclerView.ViewHolder{

        public ImageView image_profile, post_image, like, comment, save;
        public TextView username, likes, publisher, description, comments;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);

            image_profile = itemView.findViewById(R.id.image_profile);
            post_image = itemView.findViewById(R.id.post_image);
            like = itemView.findViewById(R.id.like);
            comment = itemView.findViewById(R.id.comment);
            save = itemView.findViewById(R.id.save);
            username = itemView.findViewById(R.id.username);
            likes = itemView.findViewById(R.id.likes);
            publisher = itemView.findViewById(R.id.publisher);
            description = itemView.findViewById(R.id.description);
            comments = itemView.findViewById(R.id.comments);
        }
    }
    private void publisherInfo (final ImageView image_profile, final TextView username, final TextView publisher, String userid){
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Users").child(userid);
        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                User user = dataSnapshot.getValue(User.class);
                Glide.with(mContext).load(user.getImageurl()).into(image_profile);
                username.setText(user.getUsername());
                publisher.setText(user.getUsername());
            }

            @Override
            public void onCancelled( DatabaseError databaseError) {

            }
        });
    }
}