Issue 4227 - Add some checks to consider edge conditions when element hiding stylesheet is applied

lib/child/elemHide.js

Index: lib/child/elemHide.js
===================================================================
--- a/lib/child/elemHide.js
+++ b/lib/child/elemHide.js
@@ -328,27 +328,44 @@ let observer = {
     port.emitWithResponse("elemhideEnabled", {
       frames: getFrames(subject),
       isPrivate: isPrivate(subject)
     }).then(({
       enabled, contentType, docDomain, thirdParty, location, filter,
       filterType
     }) =>
     {
+      if (Cu.isDeadWrapper(subject))
+      {
+        // We are too late, the window is gone already.
+        return;
+      }
+
       if (enabled)
       {
         if (!this.sheet)
         {
           this.sheet = Utils.styleService.preloadSheet(this.styleURL,
               Ci.nsIStyleSheetService.USER_SHEET);
         }
 
         let utils = subject.QueryInterface(Ci.nsIInterfaceRequestor)
                            .getInterface(Ci.nsIDOMWindowUtils);
-        utils.addSheet(this.sheet, Ci.nsIStyleSheetService.USER_SHEET);
+        try
+        {
+          utils.addSheet(this.sheet, Ci.nsIStyleSheetService.USER_SHEET);
+        }
+        catch (e)
+        {
+          // Ignore NS_ERROR_ILLEGAL_VALUE - it will be thrown if we try to add
+          // the stylesheet multiple times to the same document (the observer
+          // will be notified twice for some documents).
+          if (e.result != Cr.NS_ERROR_ILLEGAL_VALUE)
+            throw e;
+        }
       }
       else if (filter)
       {
         RequestNotifier.addNodeData(subject.document, subject.top, {
           contentType, docDomain, thirdParty, location, filter, filterType
         });
       }
     });