Bug 1258333 - Prevent errors when unhighlighting when the toolbox is closed; r=zer0

MozReview-Commit-ID: HUhCsqiVtEd

--HG--
extra : rebase_source : b4e18740d646192e0796ac1fd95458d8846311e4

devtools/client/framework/toolbox-highlighter-utils.js

@@ -268,7 +268,11 @@ exports.getHighlighterUtils = function(toolbox) {
       yield toolbox.highlighter.hideBoxModel();
     }
 
+    // unhighlight is called when destroying the toolbox, which means that by
+    // now, the toolbox reference might have been nullified already.
+    if (toolbox) {
       toolbox.emit("node-unhighlight");
+    }
   });
 
   /**