Bug 934487 - Fix "TypeError: this.markup is undefined" in devtools b-c runs; r=pbrosset

browser/devtools/inspector/inspector-panel.js

@@ -358,6 +358,9 @@ InspectorPanel.prototype = {
 
       this._initMarkup();
       this.once("markuploaded", () => {
+        if (this._destroyPromise) {
+          return;
+        }
         this.markup.expandNode(this.selection.nodeFront);
         this.setupSearchBox();
         this.emit("new-root");