Selected Item is not showing in Andoid spinner

Question

I'm new in android programmin so,please pardon if i did silly mistake

I want the list of users from Users collection and display in the spinner, so i can select i've tried almost everything but didn't resolve the issue, where is the problem? problem in my java code or in xml file? #Here is my XMl:#

           <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:orientation="horizontal">

            <RelativeLayout
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:layout_weight="2">


                <TextView
                    android:layout_width="match_parent"
                    android:text="Assigned User"
                    android:textStyle="bold"
                    android:textAppearance="@style/TextAppearance.AppCompat.Medium"
                    android:layout_margin="15dp"
                    android:layout_height="wrap_content"/>
            </RelativeLayout>
            <RelativeLayout
                android:layout_weight="4"
                android:layout_height="wrap_content"
                android:layout_width="0dp">

                <Spinner
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:layout_margin="10dp"
                    android:textColor="@color/black"
                    android:spinnerMode="dropdown"
                    android:id="@+id/edt_assigned_user_task"
                    />
            </RelativeLayout>

        </LinearLayout>

#Here is my code ---># when i select the item from the list the selected item dissappeared

spinnerUser = myview.findViewById(R.id.edt_assigned_user_task);
                spinnerProject =myview.findViewById(R.id.edt_project_task);
                populateSpinnerUser();



private void populateSpinnerUser() {
        final List<String> mUserList = new ArrayList<>();
        db.collection("Users").get()
                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        if(task.isSuccessful()){
                            for(DocumentSnapshot documentSnapshot: task.getResult()){
                                Log.d("TAG", documentSnapshot.getId() + " => " + documentSnapshot.getData());
                                String user = documentSnapshot.getString("uID");
                                mUserList.add(user);
                            }
                        }
                        adapter.notifyDataSetChanged();
                    }

                });

    ArrayAdapter<String> adapter = new ArrayAdapter<>(getApplicationContext(),
            android.R.layout.simple_spinner_item, mUserList);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinnerUser.setAdapter(adapter);

}




spinnerUser.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
                    @Override
                    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
                        String userName = parent.getItemAtPosition(position).toString();
                        spinnerUser.setSelection(position);
                        Toast.makeText(parent.getContext(),userName,Toast.LENGTH_SHORT);
                    }

                    @Override
                    public void onNothingSelected(AdapterView<?> parent) {

                    }
                });