Error on a null object reference in android studio, how to fix?

Question

Java

Hi, I ran into an error, I've already looked through everything, I've redone everything, I can't do anything.
Gives an error message :

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.google.firebase.firestore.QuerySnapshot.getDocumentChanges()' on a null object reference

Error logs:

Process: com.vuradoapps.mycity, PID: 8801
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.google.firebase.firestore.QuerySnapshot.getDocumentChanges()' on a null object reference
        at com.vuradoapps.mycity.Tasks_mini_app_Activity$3.onEvent(Tasks_mini_app_Activity.java:86)
        at com.vuradoapps.mycity.Tasks_mini_app_Activity$3.onEvent(Tasks_mini_app_Activity.java:82)
        at com.google.firebase.firestore.Query.lambda$addSnapshotListenerInternal$2$com-google-firebase-firestore-Query(Query.java:1131)
        at com.google.firebase.firestore.Query$$ExternalSyntheticLambda2.onEvent(Unknown Source:6)
        at com.google.firebase.firestore.core.AsyncEventListener.lambda$onEvent$0$com-google-firebase-firestore-core-AsyncEventListener(AsyncEventListener.java:42)
        at com.google.firebase.firestore.core.AsyncEventListener$$ExternalSyntheticLambda0.run(Unknown Source:6)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8653)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)

The code with the activity that the logs point to:

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.ItemTouchHelper;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;

import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.firebase.firestore.DocumentChange;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;
import com.google.firebase.firestore.ListenerRegistration;
import com.google.firebase.firestore.Query;
import com.google.firebase.firestore.QuerySnapshot;
import com.vuradoapps.mycity.Adapter.ToDoAdapter;
import com.vuradoapps.mycity.Model.ToDoModel;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;


public class Tasks_mini_app_Activity extends AppCompatActivity implements OnDialogChooce{

    private RecyclerView recyclerView;
    private FloatingActionButton mFab;
    private FirebaseFirestore firebaseFirestore;
    private ToDoAdapter adapter;
    private List<ToDoModel> mList;
    private Query query;
    private ListenerRegistration listenerRegistration;

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

        Toolbar toolbar = findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                onBackPressed();
            }
        });

        recyclerView = findViewById(R.id.recycleView);
        mFab = findViewById(R.id.floatingActionButton);
        firebaseFirestore = FirebaseFirestore.getInstance();

        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(Tasks_mini_app_Activity.this));

        mFab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Add_new_task.newInstance().show(getSupportFragmentManager(), Add_new_task.TAG);
            }
        });

        mList = new ArrayList<>();
        adapter = new ToDoAdapter(Tasks_mini_app_Activity.this, mList);

        ItemTouchHelper itemTouchHelper = new ItemTouchHelper(new TouchHelper(adapter));
        itemTouchHelper.attachToRecyclerView(recyclerView);

        recyclerView.setAdapter(adapter);
        showData();
    }

    private  void showData(){
        query = firebaseFirestore.collection("task").orderBy("time" , Query.Direction.DESCENDING);

        listenerRegistration = query.addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot value, @Nullable FirebaseFirestoreException error) {

                for (DocumentChange documentChange : value.getDocumentChanges()){
                    if (documentChange.getType() == DocumentChange.Type.ADDED){
                        String id = documentChange.getDocument().getId();
                        ToDoModel toDoModel = documentChange.getDocument().toObject(ToDoModel.class).withId(id);

                        mList.add(toDoModel);
                        adapter.notifyDataSetChanged();
                    }
                }
                listenerRegistration.remove();


            }
        });
    }

    @Override
    public void onDialogClose(DialogInterface dialogInterface) {
        mList.clear();
        showData();
        adapter.notifyDataSetChanged();
    }
}

I checked the lines, but I didn't find anything, the lines that the logs point to, they are both here:

private  void showData(){
        query = firebaseFirestore.collection("task").orderBy("time" , Query.Direction.DESCENDING);

        listenerRegistration = query.addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot value, @Nullable FirebaseFirestoreException error) {

                for (DocumentChange documentChange : value.getDocumentChanges()){
                    if (documentChange.getType() == DocumentChange.Type.ADDED){
                        String id = documentChange.getDocument().getId();
                        ToDoModel toDoModel = documentChange.getDocument().toObject(ToDoModel.class).withId(id);

                        mList.add(toDoModel);
                        adapter.notifyDataSetChanged();
                    }
                }
                listenerRegistration.remove();


            }
        });
    }

Answer 1

you have the value parameter in onEvent marked as Nullable, i.e. it is assumed that null can arrive
there. However, you boldly pull value.getDocumentChanges() without any checks.
Naturally, in a situation when null arrives in onEvent, everything falls down for you.

Answer 2

https://stackoverflow.com/questions/52791208/fireb...
an error or a normal response arrives in response.
If the error is not zero, then we process it.