[Android] Fix flyout behaviour switching exception

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue18161.cs

@@ -0,0 +1,25 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue18161 : _IssuesUITest
+{
+	public Issue18161(TestDevice device) 
+		: base(device)
+	{ }
+
+	public override string Issue => "Toggling FlyoutLayoutBehavior on Android causes the app to crash";
+
+	[Test]
+	[Category(UITestCategories.FlyoutPage)]
+	public void NoExceptionShouldBeThrown()
+	{
+		App.WaitForElement("ToggleBehaviour");
+		App.Tap("ToggleBehaviour");
+		App.Tap("ToggleBehaviour");
+
+		//The test passes if no exception is thrown
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue20858.cs

@@ -0,0 +1,28 @@
+#if ANDROID || IOS
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue20858 : _IssuesUITest
+{
+	public Issue20858(TestDevice device) 
+		: base(device)
+	{ }
+
+	public override string Issue => "FlyoutPage Android app crashing on orientation change when flyout is open";
+
+	[Test]
+	[Category(UITestCategories.FlyoutPage)]
+	public void NoExceptionShouldBeThrown()
+	{
+		App.SetOrientationPortrait();
+		App.WaitForElement("OpenFlyout");
+		App.Tap("OpenFlyout");
+		App.SetOrientationLandscape();
+
+		//The test passes if no exception is thrown
+	}
+}
+#endif

src/Controls/tests/TestCases/Issues/Issue18161.xaml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<FlyoutPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+            xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+            x:Class="Maui.Controls.Sample.Issues.Issue18161">
+  <FlyoutPage.Flyout>
+    <ContentPage Title="The FlyOut">
+      <StackLayout>
+          <Label Text="This is the flyout" />
+          <Button Text="Change Flyout behaviour" AutomationId="ToggleBehaviour" Clicked="ToggleBehaviour_Clicked" />
+        </StackLayout>
+    </ContentPage>
+  </FlyoutPage.Flyout>
+  <FlyoutPage.Detail>
+    <NavigationPage>
+      <x:Arguments>
+        <ContentPage Title="The Detail">
+          <StackLayout>
+            <Label Text="The detail page." />
+          </StackLayout>
+        </ContentPage>
+      </x:Arguments>
+    </NavigationPage>
+  </FlyoutPage.Detail>
+</FlyoutPage>

src/Controls/tests/TestCases/Issues/Issue18161.xaml.cs

@@ -0,0 +1,30 @@
+using System;
+using Microsoft.Maui;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+using Microsoft.Maui.Devices;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 18161, "Toggling FlyoutLayoutBehavior on Android causes the app to crash", PlatformAffected.Android)]
+	public partial class Issue18161 : FlyoutPage, IFlyoutPageController, IFlyoutView
+	{
+		public Issue18161()
+		{
+			InitializeComponent();
+			this.IsPresented = true;
+		}
+
+		public void ToggleBehaviour_Clicked(object sender, EventArgs e)
+		{
+			FlyoutLayoutBehavior = FlyoutLayoutBehavior == FlyoutLayoutBehavior.Split 
+				? FlyoutLayoutBehavior.Popover 
+				: FlyoutLayoutBehavior.Split;
+		}
+
+		bool IFlyoutPageController.ShouldShowSplitMode => FlyoutLayoutBehavior == FlyoutLayoutBehavior.Split;
+
+		double IFlyoutView.FlyoutWidth => 100;
+	}
+}

src/Controls/tests/TestCases/Issues/Issue20858.xaml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<FlyoutPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+            xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+            x:Class="Maui.Controls.Sample.Issues.Issue20858">
+  <FlyoutPage.Flyout>
+    <ContentPage Title="The FlyOut">
+      <Label Text="This is the flyout" />
+    </ContentPage>
+  </FlyoutPage.Flyout>
+  <FlyoutPage.Detail>
+    <NavigationPage>
+      <x:Arguments>
+        <ContentPage Title="The Detail">
+          <StackLayout>
+            <Label Text="Please rotate the device to portrait mode to test. Open the flyout and then rotate device to landscape." />
+            <Button Text="Open the Flyout to begin testing" AutomationId="OpenFlyout" Clicked="OpenFlyout_Clicked" />
+          </StackLayout>
+        </ContentPage>
+      </x:Arguments>
+    </NavigationPage>
+  </FlyoutPage.Detail>
+</FlyoutPage>

src/Controls/tests/TestCases/Issues/Issue20858.xaml.cs

@@ -0,0 +1,24 @@
+using System;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+using Microsoft.Maui.Devices;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 20858, "FlyoutPage Android app crashing on orientation change when flyout is open", PlatformAffected.Android)]
+	public partial class Issue20858 : FlyoutPage, IFlyoutPageController
+	{
+		public Issue20858()
+		{
+			InitializeComponent();
+		}
+
+		public void OpenFlyout_Clicked(object sender, EventArgs e)
+		{
+			IsPresented = true;
+		}
+
+		bool IFlyoutPageController.ShouldShowSplitMode => FlyoutLayoutBehavior == FlyoutLayoutBehavior.Split;
+	}
+}

src/Core/src/Handlers/FlyoutView/FlyoutViewHandler.Android.cs

@@ -277,6 +277,9 @@ void UpdateFlyoutBehavior()
 			var behavior = VirtualView.FlyoutBehavior;
 			if (_detailViewFragment?.DetailView?.Handler?.PlatformView == null)
 				return;
+
+			// Important to create the layout views before setting the lock mode
+			LayoutViews();
 
 			switch (behavior)
 			{
@@ -289,8 +292,6 @@ void UpdateFlyoutBehavior()
 					DrawerLayout.SetDrawerLockMode(VirtualView.IsGestureEnabled ? DrawerLayout.LockModeUnlocked : DrawerLayout.LockModeLockedClosed);
 					break;
 			}
-
-			LayoutViews();
 		}
 
 		protected override void ConnectHandler(View platformView)