firebasedatabase致命异常：main java.lang.NullPointerException

Question

我是Android和Firebase的新手，我试图从firease实时数据库中检索数据，但一次又一次面对java.lang.NullPointerException

 btnview.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                firebaseDatabase.addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                           String firstname=dataSnapshot.child("name").getValue().toString();
                           String cityname=dataSnapshot.child("city").getValue().toString();
                           name.setText(firstname);
                           city.setText(cityname);
                    }

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

                    }
                });
            }
        });

Logcat

07-15 16:45:33.049 3537-3537/com.example.newfireabseapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.newfireabseapp, PID: 3537
    java.lang.NullPointerException
        at com.example.newfireabseapp.MainActivity$2$1.onDataChange(MainActivity.java:63)
        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$1.run(com.google.firebase:firebase-database@@16.0.4:55)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5590)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1280)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1096)
        at dalvik.system.NativeStart.main(Native Method)

这是我的数据库结构

enter image description here