Issue when using multiple ApplicationEventHandler

When using multiple ApplicationEventHandler the order can not always be specified. This allows the the default controller to be specified in another startup function.

Author: revolution42

src/Our.Umbraco.DocTypeGridEditor/Bootstrap.cs

@@ -14,7 +14,10 @@ protected override void ApplicationStarting(UmbracoApplicationBase umbracoApplic
         {
             GlobalFilters.Filters.Add(new DocTypeGridEditorPreviewAttribute());
 
-            DefaultDocTypeGridEditorSurfaceControllerResolver.Current = new DefaultDocTypeGridEditorSurfaceControllerResolver();
+            if (!DefaultDocTypeGridEditorSurfaceControllerResolver.HasCurrent)
+            {
+                DefaultDocTypeGridEditorSurfaceControllerResolver.Current = new DefaultDocTypeGridEditorSurfaceControllerResolver();
+            }
         }
 
         protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
@@ -47,4 +50,4 @@ private void ExpireContentTypeCache(IContentTypeService sender, SaveEventArgs<IC
             }
         }
     }
-}
+}