[Android, iOS] Throw exceptions consistently for invalid StaticResource references to prevent relaunch crashes

src/Controls/src/Xaml/ApplyPropertiesVisitor.cs

@@ -281,10 +281,8 @@ void ProvideValue(ref object value, ElementNode node, object source, XmlName pro
 			}
 			catch (Exception e)
 			{
-				if (Context.ExceptionHandler != null)
-					Context.ExceptionHandler(e);
-				else
-					throw e;
+				Context.ExceptionHandler?.Invoke(e);
+				throw;
 			}
 		}
 

src/Controls/src/Xaml/MarkupExtensions/StaticResourceExtension.cs

@@ -30,16 +30,15 @@ public object ProvideValue(IServiceProvider serviceProvider)
 				&& !TryGetApplicationLevelResource(Key, out resource, out resourceDictionary))
 			{
 				var xmlLineInfo = serviceProvider.GetService(typeof(IXmlLineInfoProvider)) is IXmlLineInfoProvider xmlLineInfoProvider ? xmlLineInfoProvider.XmlLineInfo : null;
+				var ex = new XamlParseException($"StaticResource not found for key {Key}", xmlLineInfo);
 				if (Controls.Internals.ResourceLoader.ExceptionHandler2 is var ehandler && ehandler != null)
 				{
-					var ex = new XamlParseException($"StaticResource not found for key {Key}", xmlLineInfo);
 					var rootObjectProvider = (IRootObjectProvider)serviceProvider.GetService(typeof(IRootObjectProvider));
 					var root = rootObjectProvider.RootObject;
 					ehandler.Invoke((ex, XamlFilePathAttribute.GetFilePathForObject(root)));
-					return null;
 				}
-				else
-					throw new XamlParseException($"StaticResource not found for key {Key}", xmlLineInfo);
+				// Throw an exception when the key is not found
+				throw ex;
 			}
 
 			Diagnostics.ResourceDictionaryDiagnostics.OnStaticResourceResolved(resourceDictionary, Key, valueProvider.TargetObject, valueProvider.TargetProperty);

src/Controls/tests/DeviceTests/Xaml/StaticResourceTests.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+using Xunit;
+
+namespace Microsoft.Maui.DeviceTests;
+
+[Category(TestCategory.Xaml)]
+public class StaticResourceTests
+{
+    [Fact("Issue #23903: Missing ControlTemplate with exception handler should throw")]
+    [RequiresUnreferencedCode("XAML parsing may require unreferenced code")]
+    public void MissingControlTemplate_WithExceptionHandler_ShouldThrow()
+    {
+        // Issue #23903: StaticResourceExtension should always throw when resource is not found,
+        // even when an exception handler is present (for debug/hot reload scenarios).
+        // This prevents the app from crashing when relaunching.
+
+        Controls.Internals.ResourceLoader.ExceptionHandler2 = (ex) => { };
+
+        var xaml = """
+			<?xml version="1.0" encoding="UTF-8"?>
+			<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+			             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+			             ControlTemplate="{StaticResource InvalidTemplate}">
+			</ContentPage>
+			""";
+
+        var page = new ContentPage();
+
+        // Should throw an exception even with handler present
+        bool exceptionThrown = false;
+        try
+        {
+            page.LoadFromXaml(xaml);
+        }
+        catch (Exception)
+        {
+            exceptionThrown = true;
+        }
+
+        Assert.True(exceptionThrown, "Expected an exception to be thrown for missing ControlTemplate");
+    }
+}