fix

src/Avalonia.Controls/Platform/PlatformManager.cs

@@ -29,7 +29,7 @@ namespace Avalonia.Controls.Platform
 
             if (platform == null)
             {
-                throw new Exception("Could not CreateWindow(): IWindowingPlatform is not registered.");
+                throw new Exception("Could not CreateTrayIcon(): IWindowingPlatform is not registered.");
             }
 
             return s_designerMode ? null : platform.CreateTrayIcon();
@@ -45,7 +45,7 @@ namespace Avalonia.Controls.Platform
                 throw new Exception("Could not CreateWindow(): IWindowingPlatform is not registered.");
             }
 
-            return s_designerMode ? (IWindowImpl)platform.CreateEmbeddableWindow() : platform.CreateWindow();
+            return s_designerMode ? platform.CreateEmbeddableWindow() : platform.CreateWindow();
         }
 
         public static IWindowImpl CreateEmbeddableWindow()