Why isn't the mapView displayed in the fragment when navigating to it?

Question

Android

Hello everybody!
This is my first time making an app with maps. The bottom line is this: there is a Navigation Drawer and a few (so far) fragments. Before that, I used maps through Activity, now I switched to Fragment and then added mapView in the xml markup of this fragment: as a result, the map is not displayed when you click on the Menu Item element (direct navigation), the application itself on the smartphone does not crash, just a white screen. On the map in the future I'm going to add labels from SQLlite, so I need the ability to manipulate the code in the map class.
Markup:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.google.android.gms.maps.MapView
        android:id="@+id/mapView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.498"
        tools:ignore="MissingConstraints" />
</android.support.constraint.ConstraintLayout>

Class code:

import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.MapView;
import com.google.android.gms.maps.OnMapReadyCallback;

public class CustomMapFragment extends Fragment implements OnMapReadyCallback {

    private MapView mapView;


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_map, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Activity activity = this.getActivity();
        mapView = activity.findViewById(R.id.mapView);
        mapView.getMapAsync(this);
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {

    }
}

Manifest Permissions:

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="com.google.android.provides.gsf.permission.READ_GSERVICES" />

Answer 1

<com.google.android.gms.maps.MapView
android:layout_width="0dp"
android:layout_height="0dp"

Is this the norm in ConstrintLayout ?