Bug 1229195: ignore messages when content is null. r=Standard8

browser/base/content/social-content.js

@@ -64,6 +64,11 @@ SocialErrorListener = {
   },
 
   receiveMessage(message) {
+    if (!content) {
+      Cu.reportError("Message received whilst `content` is null: " + message.name);
+      return;
+    }
+
     let document = content.document;
 
     switch (message.name) {