Fixed Styles preview substitute

src/Avalonia.DesignerSupport/DesignWindowLoader.cs

@@ -54,7 +54,7 @@ namespace Avalonia.DesignerSupport
                             }
                         };
                 }
-                if (loaded is Application)
+                else if (loaded is Application)
                     control = new TextBlock {Text = "Application can't be previewed in design view"};
                 else
                     control = (Control) loaded;
@@ -73,4 +73,4 @@ namespace Avalonia.DesignerSupport
             return window;
         }
     }
-}
+}