尽管实例化了变量，但仍获得了空点异常

Question

我有一个返回NullPoint异常的类。我没有得到的是，有关的变量已被实例化。我不明白为什么它返回为Null。必须使用对象模型填充modelList。然后，这个modelList是我在适配器类中的参数以及另一个Activity。该modelList包含从Firestore中选取的数据值，应该对其进行检索才能在Recyclerview中显示。

Java类

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.RecyclerView;

import android.os.Bundle;

import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.QueryDocumentSnapshot;
import com.google.firebase.firestore.QuerySnapshot;

import java.util.List;

public class ViewRegisteredProperty extends AppCompatActivity {


    RecyclerView recyclerView;
    List<ModelOwnerPropertyList> modelList;

    FirebaseFirestore db;

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

        //instance of firebasefirestore
        db = FirebaseFirestore.getInstance();


        showData();
    }

    public void showData(){
        db.collection("Companies")
                .get()
                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        for (QueryDocumentSnapshot document : task.getResult()) {

                            ModelOwnerPropertyList model = new ModelOwnerPropertyList(document.getString("Address"),
                                    document.getString("CompanyName"),
                                    document.getString("TotalTenants"),
                                    document.getString("ownerName"));
                            modelList.add(model);
                        }
                    }
                })
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Toast.makeText(getApplicationContext(), "No data collected", Toast.LENGTH_SHORT).show();
                    }
                });
    }
    public void getData(){

        db.collection("Companies")
                .get()
                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        if (task.isSuccessful()) {
                            for (QueryDocumentSnapshot document : task.getResult()) {

                                ModelOwnerPropertyList model = new ModelOwnerPropertyList(document.getString("Address"),
                                        document.getString("CompanyName"),
                                        document.getString("TotalTenants"),
                                        document.getString("ownerName"));
                                modelList.add(model);
                                }
                            MyAdapter adapter = new MyAdapter(ViewRegisteredProperty.this ,modelList);
                            recyclerView.setAdapter(adapter);
                        }

                    }
                }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                Toast.makeText(getApplicationContext(), "No data collected", Toast.LENGTH_SHORT).show();
            }
        });
    }
}

StackTrace

02-21 17:06:33.215 28076-28076/com.example.rent E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.rent, PID: 28076
    java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference
        at com.example.rent.ViewRegisteredProperty$2.onComplete(ViewRegisteredProperty.java:57)
        at com.google.android.gms.tasks.zzj.run(Unknown Source)
        at android.os.Handler.handleCallback(Handler.java:815)
        at android.os.Handler.dispatchMessage(Handler.java:104)
        at android.os.Looper.loop(Looper.java:227)
        at android.app.ActivityThread.main(ActivityThread.java:6102)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:961)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:822)