用户名检查工具无效

Question

RegistrationActivity.java   

 btnRegister.setOnClickListener(new View.OnClickListener() {
                Boolean flag = false;
                @Override
                public void onClick(View view) {
                    Firebase ref = new Firebase(Config.FIREBASE_USERS_URL);

                           String userName  = edtRegistrationUsername.getText().toString().trim();


                    final User user = new User();

                    user.setUserName(userName,RegistrationActivity.this);




                    ref.addValueEventListener(new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot snapshot) {
                            for (DataSnapshot postSnapshot : snapshot.getChildren()) {
                                //Getting the data from snapshot
                                User user1 = postSnapshot.getValue(User.class);

                                if(user1.getUserName().equals(user.getUserName())){
                                    Toast.makeText(RegistrationActivity.this,"That username is already taken",Toast.LENGTH_LONG);
                                    flag = false;
                                }else{
                                    flag = true;
                                }
                            }
                        }

                        @Override
                        public void onCancelled(FirebaseError firebaseError) {
                            System.out.println("The read failed: " + firebaseError.getMessage());
                        }
                    });

                    if(flag == true){
                        ref.push().setValue(user);
                    }


                }
            });

如果用户名存在，我想检查firebase数据库，否则在firebase中添加用户名。

当我输入用户名并单击注册时，程序崩溃并显示错误：

    Task exception on worker thread: java.lang.IncompatibleClassChangeError: The method 'java.io.File android.support.v4.content.ContextCompat.getNoBackupFilesDir(android.content.Context)' was expected to be of type virtual but instead was found to be of type direct (declaration of 'com.google.firebase.iid.zzg'

我希望有人可以帮我解决问题。谢谢！