Use node.contains instead of checking element.parentNode in mutations

We used to check if an element had a null parent, but it is possible to
have an existing parent but still be out of dom (with a null grand*-parent)

src/js/editor/mutation-handler.js

@@ -116,7 +116,8 @@ export default class MutationHandler {
         break;
     }
 
-    let attachedNodes = filter(nodes, node => !!node.parentNode);
+    let element = this.editor.element;
+    let attachedNodes = filter(nodes, node => element.contains(node));
     return attachedNodes;
   }