FindViewById<Toolbar>(Resource.Id.toolbar) return null - xamarin.forms

Question

I use this example:
https://github.com/UdaraAlwis/Xamarin-Playground/tree/master/XFNavBarBackBtnClickOverride.
But FindViewById(Resource.Id.toolbar) return null.
In the new project this example working. On my project not working. What else can I check?
MainAcitvity.cs
protected override void OnCreate(Bundle bundle)
{
TabLayoutResource = Resource.Layout.Tabbar;
ToolbarResource = Resource.Layout.Toolbar;
base.OnCreate(bundle);
Xamarin.Essentials.Platform.Init(this, bundle);
Forms.Init(this, bundle);
CrossCurrentActivity.Current.Init(this, bundle);
Rg.Plugins.Popup.Popup.Init(this, bundle);
ToastNotification.Init(this);
Xamarin.FormsMaps.Init(this, bundle);
DependencyService.Register<ToastNotification>();
LoadApplication(new App());
Toolbar toolbar = FindViewById<Toolbar>(Resource.Id.toolbar);
SetSupportActionBar(toolbar);
}
Toolbar.xml
<android.support.v7.widget.Toolbar
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/toolbar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="?attr/colorPrimary"
android:theme="#style/ThemeOverlay.AppCompat.Dark.ActionBar"/>
styles.xml
<?xml version="1.0" encoding="utf-8" ?>
<resources>
<style name="MainTheme" parent="MainTheme.Base">
</style>
<style name="MainTheme.Base" parent="Theme.AppCompat.Light.DarkActionBar">
<item name="windowNoTitle">true</item>
<item name="windowActionBar">false</item>
<item name="colorPrimary">#2196F3</item>
<item name="colorPrimaryDark">#1976D2</item>
<item name="colorAccent">#172239</item>
<item name="windowActionModeOverlay">true</item>
<item name="android:datePickerDialogTheme">#style/AppCompatDialogStyle</item>
<item name="android:textAllCaps">false</item>
</style>
<style name="AppCompatDialogStyle" parent="Theme.AppCompat.Light.Dialog">
<item name="colorAccent">#FF4081</item>
</style>
<color name="ListViewHighlighted">#layout/gradient</color>
</resources>

Answer 1

The following line
Toolbar toolbar = FindViewById<Toolbar>(Resource.Id.toolbar);
When you didn't define the toolbar as Android.Support.V7.Widget.Toolbar , it will been init as Android..Widget.Toolbar.So the method FindViewById will never find the toolbar in xml .
Instead it as the following line.
Android.Support.V7.Widget.Toolbar toolbar = this.FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
And you need to set the MainPage as a NavigationPage in app.xaml.cs .
MainPage = new NavigationPage(new xxxPage());