Merge pull request #6219 from AvaloniaUI/fixes/sandbox-designer

Make the designer work in the sandbox project.

samples/Sandbox/Program.cs

@@ -4,12 +4,12 @@ namespace Sandbox
 {
     public class Program
     {
-        static void Main(string[] args)
-        {
+        static void Main(string[] args) => BuildAvaloniaApp()
+            .StartWithClassicDesktopLifetime(args);
+
+        public static AppBuilder BuildAvaloniaApp() =>
             AppBuilder.Configure<App>()
                 .UsePlatformDetect()
-                .LogToTrace()
-                .StartWithClassicDesktopLifetime(args);
-        }
+                .LogToTrace();
     }
 }