[release/8.0.1xx-sr9] NullReferenceException when setting BarBackgroundColor for a NavigationPage - fix (#25251)

* Added a null check

* Added a UI test

* Removed an xaml file for ui test

* Update NavigationPageToolbar.cs

* Improvements

* Update NavigationPageToolbar.cs

---------

Co-authored-by: Jakub Florkowski <kubaflo123@gmail.com>

Author: PureWeen

src/Controls/src/Core/NavigationPage/NavigationPageToolbar.cs

@@ -33,7 +33,6 @@ public NavigationPageToolbar(Maui.IElement parent, Page rootPage) : base(parent)
 			_toolbarTracker.CollectionChanged += OnToolbarItemsChanged;
 			RootPage = rootPage;
 			_toolbarTracker.PageAppearing += OnPageAppearing;
-			_toolbarTracker.PagePropertyChanged += OnPagePropertyChanged;
 			_toolbarTracker.Target = RootPage;
 		}
 
@@ -71,6 +70,7 @@ void OnPageAppearing(object sender, EventArgs e)
 			if (sender is not ContentPage cp)
 				return;
 
+			_toolbarTracker.PagePropertyChanged -= OnPagePropertyChanged;
 			_currentPage = cp;
 			_currentNavigationPage = _currentPage.FindParentOfType<NavigationPage>();
 
@@ -112,6 +112,7 @@ void OnPageAppearing(object sender, EventArgs e)
 			_hasAppeared = true;
 
 			ApplyChanges(_currentNavigationPage);
+			_toolbarTracker.PagePropertyChanged += OnPagePropertyChanged;
 		}
 
 		// This is to catch scenarios where the user

src/Controls/tests/TestCases.HostApp/Issues/Issue25114.cs

@@ -0,0 +1,40 @@
+namespace Maui.Controls.Sample.Issues;
+using Microsoft.Maui.Controls;
+
+[Issue(IssueTracker.Github, 25114, "NullReferenceException when setting BarBackgroundColor for a NavigationPage", PlatformAffected.All)]
+public class Issue25114Flyout : FlyoutPage
+{
+	public Issue25114Flyout()
+	{
+		var navPage = new NavigationPage(new Issue25114());
+		navPage.SetDynamicResource(NavigationPage.BarBackgroundColorProperty, "Primary");
+		Detail = navPage;
+		Flyout = new ContentPage
+		{
+			Title = "Flyout"
+		};
+	}
+}
+
+public class Issue25114 : ContentPage
+{
+	public Issue25114()
+	{
+		Content = new Button()
+		{
+			AutomationId = "button",
+			HeightRequest = 100,
+			Text = "Change the Navigation Bar's Color",
+			Command = new Command(() =>
+			{
+				Application.Current.Resources["Primary"] = "#0000FF";
+			})
+		};
+	}
+
+	protected override void OnAppearing()
+	{
+		base.OnAppearing();
+		Application.Current.Resources["Primary"] = "#00FF00";
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue25114.cs

@@ -0,0 +1,23 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue25114 : _IssuesUITest
+{
+	public Issue25114(TestDevice testDevice) : base(testDevice)
+	{
+	}
+
+	public override string Issue => "NullReferenceException when setting BarBackgroundColor for a NavigationPage";
+
+	[Test]
+	[Category(UITestCategories.FlyoutPage)]
+	public void NoExceptionShouldBeThrownWhenChangingNavigationBarColor()
+	{
+		App.WaitForElement("button");
+		App.Tap("button");
+	}
+
+}