我的addValueEventListener直接转到OnCancelled而不是OnDataChange

Question

我的EventListener跳过OnDataChange到OnCancelled。 Android Studio在编译应用程序时未显示任何错误。但是，当我尝试从Firebase检索数据时出现问题。

uploads = new ArrayList<>();
mDatabase = FirebaseDatabase.getInstance().getReference(Constants.DATABASE_PATH_UPLOADS);


//adding an event listener to fetch values
mDatabase.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        //dismissing the progress dialog
        progressDialog.dismiss();

        //iterating through all the values in database
        for (DataSnapshot postSnapshot : snapshot.getChildren()) {
            Upload upload = postSnapshot.getValue(Upload.class);
            uploads.add(upload);

        }
        //creating adapter
        adapter = new MyAdapter(getApplicationContext(), uploads);

        //adding adapter to recyclerview
        //MyAdapter adapter = new MyAdapter(new ArrayList<String>(list));
        recyclerView.setAdapter(adapter);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Toast.makeText(getApplicationContext(),"Error connecting to database", Toast.LENGTH_SHORT).show();
        //progressDialog.dismiss();
    }
});