Bug 1251959 - Fix "this.contentChatNode is null" on shutdown. r=aleth

--HG--
extra : amend_source : 9e2425d4f4bc78956f1125bc6b3ce765653ff500

chat/content/convbrowser.xml

@@ -318,8 +318,11 @@
 
             this.uninitMagicCopy();
 
+            if (this.contentChatNode) {
+              // Remove the listener only if the conversation was initialized.
               this.contentChatNode
                   .removeEventListener("load", this.onContentElementLoad, true);
+            }
           ]]>
         </body>
       </method>