Fix: sysMessageHandler not deleted (#1590)

* Fix: sysMessageHandler not deleted

#1589

* Update xmpp.js

modules/xmpp/xmpp.js

@@ -281,6 +281,7 @@ export default class XMPP extends Listenable {
             // once connected or attached we no longer need this handle, drop it if it exist
             if (this._sysMessageHandler) {
                 this.connection._stropheConn.deleteHandler(this._sysMessageHandler);
+                this._sysMessageHandler = null;
             }
 
             this.sendDiscoInfo && this.connection.jingle.getStunAndTurnCredentials();
@@ -496,8 +497,11 @@ export default class XMPP extends Listenable {
         this.sendDiscoInfo = true;
 
         if (this.connection._stropheConn && this.connection._stropheConn._addSysHandler) {
-            this._sysMessageHandler = this._onSystemMessage.bind(this);
-            this.connection._stropheConn._addSysHandler(this._sysMessageHandler, null, 'message');
+            this._sysMessageHandler = this.connection._stropheConn._addSysHandler(
+                this._onSystemMessage.bind(this),
+                null,
+                'message'
+            );
         } else {
             logger.warn('Cannot attach strophe system handler, jiconop cannot operate');
         }