Test if toggleHandTool is null before using it

Do not break the viewer if the buttons are not
available for some reason.

web/hand_tool.js

@@ -25,6 +25,9 @@ var HandTool = {
     this.handTool = new GrabToPan({
       element: options.container,
       onActiveChanged: function(isActive) {
+        if (!toggleHandTool) {
+          return;
+        }
         if (isActive) {
           toggleHandTool.title =
             mozL10n.get('hand_tool_disable.title', null, 'Disable hand tool');
@@ -36,9 +39,11 @@ var HandTool = {
           toggleHandTool.firstElementChild.textContent =
             mozL10n.get('hand_tool_enable_label', null, 'Enable hand tool');
         }
-    }
+      }
     });
-    toggleHandTool.addEventListener('click', this.handTool.toggle, false);
+    if (toggleHandTool) {
+      toggleHandTool.addEventListener('click', this.handTool.toggle, false);
+    }
     // TODO: Read global prefs and call this.handTool.activate() if needed.
   },