问幻灯片菜单布局返回左侧和内容视图的空引用

EN

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
tools:context=".RootActivity">


<com.jkb.slidemenu.SlideMenuLayout
    android:id="@+id/mainSlideMenu"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/white"
    app:slideMode="left">
    <include layout="@layout/left_menu_layout" />
    <include layout="@layout/content_layout" />
</com.jkb.slidemenu.SlideMenuLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

public class RootActivity extends AppCompatActivity {

    private SlideMenuLayout leftMenu;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_root);

        leftMenu = (SlideMenuLayout) findViewById(R.id.mainSlideMenu);

        View left = leftMenu.getSlideLeftView(); // returns null !!!

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.container, DashboardFragment.newInstance())
                    .commitNow();
        }
    }


}