Android project issue [closed]

I'm having some particular problems with my android project. In particular I have two collections, Users and Courses: the first one comprehend the list of users (teachers) which have two Strings (email and idCourse) in the second one there is the list of courses, each one relating to a user (idCourse is the same of the user), inside each course there is a list of lesson which i need to access with a specific user. I saved in an object admin its data to access to a specific Lessons collection, because in the path there is the admin id.

Courses path to Lessons Users collection

I tried to, but when i save the id in the addCompleteListener method the data get lost outside, indeed in the addSnapshotListener the value of admin.getCourseId() is null so my recyclerView results empty. If I try to copy the second method in the first one, as soon as I save the idCourse in Admin, the method works, but if I click on the lesson when i use the app, the app crashes. I know this may be a specifical issue of mine, but i would like to hear something from you about it. I'm posting my android project too. Android Project on Google Drive Thanks in advance for your help!

CLASS LezioniDocenteActivity:

public class LezioniDocenteActivity extends AppCompatActivity {

    private static final String TAG = "FireLog";
    private RecyclerView mMainList;
    private FirebaseFirestore mFirestore;
    private FirebaseAuth firebaseAuth;
    private LessonsListAdapter lessonsListAdapter;
    private List<Lesson> lessonList;
    private String course_id;

    //admin per memorizzare le info
    private final User admin = new User();

    @Override
    public void onBackPressed(){
        startActivity(new Intent(LezioniDocenteActivity.this, DocenteActivity.class));
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_lezioni_docente);

        lessonList = new ArrayList<>();
        lessonsListAdapter = new LessonsListAdapter(lessonList, getApplicationContext());

        mMainList = (RecyclerView)findViewById(R.id.lesson_list);
        mMainList.setHasFixedSize(true);
        mMainList.setLayoutManager(new LinearLayoutManager(this));
        mMainList.setAdapter(lessonsListAdapter);
        firebaseAuth = FirebaseAuth.getInstance();

        //utente in uso corrente
        final FirebaseUser user = firebaseAuth.getCurrentUser();
        admin.setEmail(user.getEmail());
        mFirestore = FirebaseFirestore.getInstance();

        mFirestore.collection("Users").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                boolean find = false;
                for (DocumentSnapshot doc : task.getResult()) {
                    String email = doc.getString("email");
                    if (email.equals(admin.getEmail())) {
                        //getting admin's courseID
                        admin.setCourseId(doc.getString("idCorso"));
                        break;
                    }
                }
            }
        });

        mFirestore.collection("Courses /" + admin.getCourseId() + "/Lessons").addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot documentSnapshots, @Nullable FirebaseFirestoreException e) {
                if(e!=null){
                    Log.d(TAG, "Error :" + e.getMessage());
                    return;
                }
                for(DocumentChange doc : documentSnapshots.getDocumentChanges()){
                    if(doc.getType() == DocumentChange.Type.ADDED){
                        Lesson lesson = doc.getDocument().toObject(Lesson.class);
                        lessonList.add(lesson);
                        lessonsListAdapter.notifyDataSetChanged();
                    }
                }
            }
        });

    }
}

CLASS LessonListAdapter:

public class LessonsListAdapter extends RecyclerView.Adapter<LessonsListAdapter.ViewHolder> {

    public List<Lesson> lessonList;
    public Context context;
    final User admin = new User();

    public LessonsListAdapter(List<Lesson> lessonList, Context context){
        this.lessonList = lessonList;
        this.context = this.context;
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.list_lessons, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull final ViewHolder holder, int position) {

        final FirebaseFirestore firebaseFirestore = FirebaseFirestore.getInstance();
        final FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();
        final FirebaseUser user = firebaseAuth.getCurrentUser();
        admin.setEmail(user.getEmail());

        holder.dayText.setText(lessonList.get(position).getLessonDate());
        holder.argText.setText(lessonList.get(position).getArgument());
        final String lessID = lessonList.get(position).lessonID;
        final int i = position;

        firebaseFirestore.collection("Users").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        boolean find = false;
                        for(DocumentSnapshot doc : task.getResult()){
                            String email = doc.getString("email");

                            if(email.equals(admin.getEmail())) {
                                find = true;
                                //getting admin's courseID
                                admin.setCourseId(doc.getString("courseId"));
                                break;
                            }
                        }
            if (find){
                holder.delete.setVisibility(View.VISIBLE);
                holder.delete.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(context, "OnClick funziona", Toast.LENGTH_SHORT).show();
                    }
                });
            } else holder.delete.setVisibility(View.GONE);
        }
    });
    }

    @Override
    public int getItemCount() {
        return lessonList.size();
    }

    public class ViewHolder extends RecyclerView.ViewHolder{

        View mView;

        public TextView dayText;
        public TextView argText;
        public ImageView delete;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            mView = itemView;
            dayText = (TextView) mView.findViewById(R.id.day_text);
            argText = (TextView) mView.findViewById(R.id.arg_text);
            delete = (ImageView) mView.findViewById(R.id.ic_delete);
        }
    }
}