Wie löse ich das Problem eines NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference ?

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        // check edit text change listener
        messageEt.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            }
            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                if (s.toString().trim().length() == 0 ) {
                    checkTypingStatus("noOne");
                } else {
                    checkTypingStatus(hisUid); // uid of receiver
                }
            }
            @Override
            public void afterTextChanged(Editable s) {
            }
        });
        readMessages();
        seenMessage();
    }
    private void seenMessage() {
        userRefForSeen = FirebaseDatabase.getInstance().getReference("Chats");
        seenListener = userRefForSeen.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    ModelChat chat = ds.getValue(ModelChat.class);
                    if ( chat.getReceiver().equals(myUid) && chat.getSender().equals(hisUid)) {
                        HashMap<String, Object> hasSeenHashMap = new HashMap<>();
                        hasSeenHashMap.put("isSeen", true);
                        ds.getRef().updateChildren(hasSeenHashMap);
                    }
                }
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }
    private void readMessages() {
        chatList = new ArrayList<>();
        DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Chats");
        dbRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                chatList.clear();
                for (DataSnapshot ds : dataSnapshot.getChildren()) {
                    ModelChat chat = ds.getValue(ModelChat.class);
                    if (chat.getReceiver().equals(myUid) ||
                            (chat.getReceiver().equals(hisUid) && chat.getSender().equals(myUid))) {
                        chatList.add(chat);
                    }
                    // adapter
                    adapterChat = new AdapterChat(ChatActivity.this, chatList, hisImage);
                    adapterChat.notifyDataSetChanged();
                    // set adapter to recyclerview
                    recyclerView.setAdapter(adapterChat);
                }
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });
    }
    private void sendMessage(String message) {
        /* "Chats" node will be created, that will contain all chats. Whenever user
         * sends message it will create a new child in "Chats" node and that child
         * will contain the following key values.
         * sender: UID of sender
         * receiver: UID of receiver
         * message: the actual message
         */
        DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();
        String timestamp = String.valueOf(System.currentTimeMillis());
        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("sender", myUid);
        hashMap.put("receiver", hisUid);
        hashMap.put("message", message);
        hashMap.put("timestamp", timestamp);
        hashMap.put("isSeen", false);
        databaseReference.child("Chats").push().setValue(hashMap);
        // reset edittext after sending message
        messageEt.setText("");
    }
    private void checkUserStatus() {
        // get current user
        FirebaseUser user = firebaseAuth.getCurrentUser();
        if (user != null) {
            // user is signed in stay here
            //set email of logged in user
            //mProfileTv.setText(user.getEmail());
            myUid = user.getUid(); // currently signed in user's uid
        } else {
            // user not signed in, go to main activity
            startActivity(new Intent(this, MainActivity.class));
            finish();
        }
    }
    private void checkOnlineStatus(String status) {
        DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Users").child(myUid);
        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("onlineStatus", status);
        // update value of onlineStatus of current user
        dbRef.updateChildren(hashMap);
    }
    private void checkTypingStatus(String typing) {
        DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Users").child(myUid);
        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("typingTo", typing);
        // update value of onlineStatus of current user
        dbRef.updateChildren(hashMap);
    }


        }
        return super.onOptionsItemSelected(item);
    }
}

public class ModelChat {
    String message, receiver, sender, timestamp;
    boolean isSeen;
    public ModelChat() {
    }
    public ModelChat(String message, String receiver, String sender, String timestamp, boolean isSeen) {
        this.message = message;
        this.receiver = receiver;
        this.sender = sender;
        this.timestamp = timestamp;
        this.isSeen = isSeen;
    }
    public String getMessage() {
        return message;
    }
    public void setMessage(String message) {
        this.message = message;
    }
    public String getReceiver() {
        return receiver;
    }
    public void setReceiver(String receiver) {
        this.receiver = receiver;
    }
    public String getSender() {
        return sender;
    }
    public void setSender(String sender) {
        this.sender = sender;
    }
    public String getTimestamp() {
        return timestamp;
    }
    public void setTimestamp(String timestamp) {
        this.timestamp = timestamp;
    }
    public boolean isSeen() {
        return isSeen;
    }
    public void setSeen(boolean seen) {
        isSeen = seen;
    }
}