Java 如何解决数据更改时空对象引用上的此错误 我的代码 readmassage(); seenmasseges(); private void seenmasseges() { userRefforseen=FirebaseDatabase.getIn

如何解决数据更改时空对象引用上的此错误 我的代码

readmassage();
seenmasseges();

private void seenmasseges() {
    userRefforseen=FirebaseDatabase.getInstance().getReference("chats");
    valueEventListener=userRefforseen.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for(DataSnapshot ds :dataSnapshot.getChildren()){
                ModulChat chat=ds.getValue(ModulChat.class);
                if(chat.getRecerve().equals(hisuide)&&chat.getSender().equals(myuid))
                {
                    HashMap<String,Object> hashMap=new HashMap<>();
                    hashMap.put("isseen",true);
                    ds.getRef().updateChildren(hashMap);
                }
            }
        }

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

        }
    });

}

private void readmassage() {

    modulChats=new ArrayList<>();
    DatabaseReference dbrf=FirebaseDatabase.getInstance().getReference().child("chats");
    dbrf.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            modulChats.clear();
            for(DataSnapshot ds :dataSnapshot.getChildren()){
                ModulChat chat=ds.getValue(ModulChat.class);
                if(chat.getRecerve().equals(myuid)&&chat.getSender().equals(hisuide)||
                        chat.getRecerve().equals(hisuide)&&chat.getSender().equals(myuid)){
                    modulChats.add(chat);

                }
                adapterChat=new AdapterChat(chatmassage.this,modulChats,hisimage);
                adapterChat.notifyDataSetChanged();
                chat_mass.setAdapter(adapterChat);
            }

        }

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

        }
    });

}

#error #

>E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.chat, PID: 6655
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
        at com.example.chat.chatmassage$4.onDataChange(chatmassage.java:173)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@17.0.0:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@17.0.0:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@17.0.0:55)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
I/Process: Sending signal. PID: 6655 SIG: 9
Process 6655 terminated.




##how to solve this please help and thanks##