Better fix for the NRE

src/Avalonia.Controls/TopLevel.cs

@@ -454,7 +454,8 @@ namespace Avalonia.Controls
             }
             else if (change.Property == ActualThemeVariantProperty)
             {
-                PlatformImpl?.SetFrameThemeVariant((PlatformThemeVariant?)change.GetNewValue<ThemeVariant?>() ?? PlatformThemeVariant.Light);
+                var newThemeVariant = change.GetNewValue<ThemeVariant?>() ?? ThemeVariant.Default;
+                PlatformImpl?.SetFrameThemeVariant((PlatformThemeVariant?)newThemeVariant ?? PlatformThemeVariant.Light);
             }
         }