Fix exception when closing color picker.

And add tests.

src/Markup/Avalonia.Markup.Xaml/Data/DynamicResourceExpression.cs

@@ -86,6 +86,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 
         private void ActualThemeVariantChanged(object? sender, EventArgs e)
         {
+            if (!IsRunning)
+                return;
+
             _themeVariant = ((IThemeVariantHost)sender!).ActualThemeVariant;
             PublishValue();
         }

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/DynamicResourceExtensionTests.cs

@@ -896,6 +896,34 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
             Assert.DoesNotContain("foo", resourceProvider.RequestedResources);
         }
 
+        [Fact]
+        public void Can_Detach_Control_With_DynamicResource_ControlTheme_That_Contains_DynamicResource()
+        {
+            using var app = UnitTestApplication.Start(TestServices.StyledWindow);
+            var xaml = $@"
+<Window xmlns='https://github.com/avaloniaui'
+    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+    RequestedThemeVariant='Light'>
+    <Window.Resources>
+        <SolidColorBrush x:Key='Blue'>Blue</SolidColorBrush>
+        <ControlTheme x:Key='MyTheme' TargetType='Button'>
+            <Setter Property='Background' Value='{{DynamicResource Blue}}'/>
+        </ControlTheme>
+    </Window.Resources>
+
+    <Button Theme='{{DynamicResource MyTheme}}'/>
+</Window>";
+
+            var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+            var target = Assert.IsType<Button>(window.Content);
+
+            window.Show();
+
+            Assert.Equal(Colors.Blue, ((ISolidColorBrush)target.Background).Color);
+
+            window.Content = null;
+        }
+
         private IDisposable StyledWindow(params (string, string)[] assets)
         {
             var services = TestServices.StyledWindow.With(

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/ControlThemeTests.cs

@@ -38,6 +38,36 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             }
         }
 
+        [Fact]
+        public void ControlTheme_Can_Be_DynamicResource()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = $@"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:u='using:Avalonia.Markup.Xaml.UnitTests.Xaml'>
+    <Window.Resources>
+        {ControlThemeXaml}
+    </Window.Resources>
+
+    <u:TestTemplatedControl Theme='{{DynamicResource MyTheme}}'/>
+</Window>";
+
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var button = Assert.IsType<TestTemplatedControl>(window.Content);
+
+                window.Show();
+
+                Assert.NotNull(button.Template);
+
+                var child = Assert.Single(button.GetVisualChildren());
+                var border = Assert.IsType<Border>(child);
+
+                Assert.Equal(Brushes.Red, border.Background);
+            }
+        }
+
         [Fact]
         public void ControlTheme_Can_Be_Set_In_Style()
         {