prevent editor open when control is not ready yet

src/vs/workbench/browser/parts/editor/editorPart.ts

@@ -264,7 +264,8 @@ export class EditorPart extends Part implements IEditorPart, IEditorGroupService
 			!input ||																		// no input
 			position === null ||															// invalid position
 			Object.keys(this.mapEditorInstantiationPromiseToEditor[position]).length > 0 ||	// pending editor load
-			this.editorGroupsControl.isDragging()												// pending editor DND
+			!this.editorGroupsControl ||													// too early
+			this.editorGroupsControl.isDragging()											// pending editor DND
 		) {
 			return TPromise.as<BaseEditor>(null);
 		}