public void onItemSelected(AdapterView<?> adapterView, View view,
                                           int position, long id) {
                    Object item = adapterView.getItemAtPosition(position);
                    NameSelected = item.toString();
                    pris = NameSelected;
                    ref = FirebaseDatabase.getInstance().getReference().child("Faculty_Database");
                    ref.orderByChild("Name").equalTo(pris).addValueEventListener(new ValueEventListener(){
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {
                            for(DataSnapshot datas: dataSnapshot.getChildren()){
                                key = datas.child("UID").getValue(String.class);
                            }
                        }

databaseReference = FirebaseDatabase.getInstance().getReference().child("Faculty_Database").child(key).child("Received"); // this is where I'm getting the error
                                Map<String, Object> mapToUpload = new HashMap<>();
                                mapToUpload.put("imageUrl", downloadUri.toString());
                                mapToUpload.put("Data", studentDetails);
                                databaseReference.child(StudentRecordIDFromServer).setValue(mapToUpload);