Adds JavaDoc comments to closeInvalidUpperNode Method

src/ui/view/GUI.java

@@ -3140,6 +3140,10 @@ public class GUI implements CategoryListener {
 		lblHint.setVisible(enable);
 	}
 	
+	/**
+	 * closes all UpperNodeTabs, that don't have a
+	 * valid UpperNode (e.g. if it was ungrouped/deleted/replaced and so on)
+	 */
 	private void closeInvalidUpperNodeTabs() {
 		/**
 		 * close bugged Tabs