Issue 2816 - Partial fix for EHH button in inspector tool, preview functionality still broken

lib/inspectorObserver.js

 /*
  * This Source Code is subject to the terms of the Mozilla Public License
  * version 2.0 (the "License"). You can obtain a copy of the License at
  * http://mozilla.org/MPL/2.0/.
  */
 
 let InspectorObserver =
 {
   init: function()
   {
@@ skipping 34 matching lines @@
 
     let tooltiptext = InspectorObserver.inspectorButtonTooltip;
     button = panelWindow.document.createElement("toolbarbutton");
     button.setAttribute("id", "ehh-inspector-toolbarbutton");
     button.setAttribute("class", "devtools-toolbarbutton");
     button.setAttribute("tooltiptext", tooltiptext);
     button.setAttribute("tabindex", "0");
     button.addEventListener("command", () =>
     {
       let node = panel.selection.nodeFront;
-      if (panel.target.form.elemhidehelper)
+      let target = panel.target;
+      if (node && target.form.elemhidehelper)
       {
-        panel.target.client.request({to: panel.target.form.elemhidehelper, type: "nodeinfo", nodeActor: node.actorID}, function(response)
+        target.client.request({
+          to: target.form.elemhidehelper,
+          type: "nodeinfo",
+          nodeActor: node.actorID
+        }, function(response)
         {
-          if ("nodeData" in response)
-          {
-            panelWindow.openDialog("chrome://elemhidehelper/content/composer.xul", "_blank",
-                                   "chrome,centerscreen,resizable,dialog=no", response.nodeData, response.host);
-          }
+          if (!response.nodeData)
+            return;
+
+          panelWindow.openDialog("chrome://elemhidehelper/content/composer.xul",
+              "_blank", "chrome,centerscreen,resizable,dialog=no",
+              response.nodeData, response.host);
         });
       }
     }, false);
 
     //Override button style for light DevTools theme
     let style = panelWindow.document.createProcessingInstruction("xml-stylesheet", 'href="chrome://elemhidehelper/skin/devToolsOverlay.css" type="text/css"');
     panelWindow.document.insertBefore(style, panelWindow.document.firstChild);
 
     inspectBtn.parentNode.insertBefore(button, inspectBtn);
   }
 };
 
 InspectorObserver.init();