[MacOS] Fix NRE whrn checking for existing MainMenu, fixes #3353 (#3511)

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -944,7 +944,7 @@
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue2858.xaml">
       <SubType>Designer</SubType>
-      <Generator>MSBuild:Compile</Generator>
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
   </ItemGroup>
 </Project>

Xamarin.Forms.Platform.MacOS/FormsApplicationDelegate.cs

@@ -29,7 +29,9 @@ namespace Xamarin.Forms.Platform.MacOS
 
 			Application.SetCurrentApplication(application);
 			_application = application;
-			_storyboardMainMenuCount = (int)NSApplication.SharedApplication.MainMenu.Count;
+
+			if(NSApplication.SharedApplication.MainMenu != null)
+				_storyboardMainMenuCount = (int)NSApplication.SharedApplication.MainMenu.Count;
 
 			application.PropertyChanged += ApplicationOnPropertyChanged;
 		}