I keep on getting "on a null object reference" while the object clearly isn't null

so i have a database that is filled with data about teachers, and when i try to open a fragment that has a recyclerView inside of it that displays all of the teachers in the database it crashes!
and what makes this a bit more illogical is when i tries to make a toast that display the size of the list returned by the viewModel observer it returns the correct amount of teachers in database, same as when i tried to make a toast that display the name of the first entry in the list, it worked just fine

i apologize if the description of my problem isn’t clear enough, am still a bit new
nevertheless here is the code to my fragment that crashes when i open it :

public class AddHoursFragment extends Fragment {
private RecyclerView recyclerView;
private AddPaymentAdapter adapter;
private SchoolViewModel schoolViewModel;
private Button btn;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View v = inflater.inflate(R.layout.fragment_add_hours, container, false);
    btn = v.findViewById(R.id.btn_add_payment);
    recyclerView = v.findViewById(R.id.rv_add_payment);
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(new LinearLayoutManager(getContext(), LinearLayoutManager.HORIZONTAL, false));
    schoolViewModel = new ViewModelProvider(this).get(SchoolViewModel.class);

    schoolViewModel.getAllTeachers().observe(getViewLifecycleOwner(), new Observer<List<Teacher>>() {
        @Override
        public void onChanged(List<Teacher> teachers) {
            adapter.setTeachers(teachers);
            btn.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    Toast.makeText(getContext(), teachers.get(0).getTeacherName()+"" , Toast.LENGTH_SHORT).show();
                }
            });
        }
    });
    recyclerView.setAdapter(adapter);

    return v;
}

here is the code of the adapter of the recyclerView inside the fragment :

public class AddPaymentAdapter extends RecyclerView.Adapter<AddPaymentAdapter.AddPaymentHolder> {
private List<Teacher> teachers = new ArrayList<>();

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

@Override
public void onBindViewHolder(@NonNull AddPaymentHolder holder, int position) {
    Teacher teacher = teachers.get(position);
        holder.checkBox.setText(teacher.getTeacherName());
}

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

public class AddPaymentHolder extends RecyclerView.ViewHolder {
    CheckBox checkBox;
    public AddPaymentHolder(@NonNull View itemView) {
        super(itemView);
        checkBox = itemView.findViewById(R.id.checkBox1);
    }
}
public void setTeachers(List<Teacher> teachers){
    this.teachers = teachers;
    notifyDataSetChanged();
}

and this is what the logcat shows when the app crashes :

example.ecolemathphysique, PID: 7128
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.ecolemathphysique.Adapters.AddPaymentAdapter.setTeachers(java.util.List)' on a null object reference
    at com.example.ecolemathphysique.AddHoursFragment$1.onChanged(AddHoursFragment.java:44)
    at com.example.ecolemathphysique.AddHoursFragment$1.onChanged(AddHoursFragment.java:41)
    

>Solution :

You have a private field:

private AddPaymentAdapter adapter;

and you call in

adapter.setTeachers(teachers);

Cannot see in this code, but you like are missing the assignment to adapter field.