feat: add warning, resolve a crash issue.

demo/Ursa.Demo/Pages/EnumSelectorDemo.axaml

@@ -12,6 +12,7 @@
     x:DataType="vm:EnumSelectorDemoViewModel"
     mc:Ignorable="d">
     <StackPanel>
+        <u:Banner Type="Warning" Header="Native AOT non-compatible" Content="EnumSelector highly relies on reflection, it is not compatible with NativeAOT compilation. " Classes="Warning"></u:Banner>
         <ToggleSwitch Name="description" Content="Show Description" />
         <TextBlock Text="Select Type" />
         <ComboBox

src/Ursa/Controls/EnumSelector/EnumSelector.cs

@@ -92,7 +92,10 @@ public class EnumSelector: TemplatedControl
         var newValue = args.NewValue.Value;
         if (newValue is null)
         {
-            SetCurrentValue(SelectedValueProperty, null);
+            if (Values?.Any() == true)
+            {
+                SetCurrentValue(SelectedValueProperty, null);
+            }
         }
         else
         {