Merge branch 'master' of github.com:thingsboard/thingsboard

netty-mqtt/src/main/java/org/thingsboard/mqtt/MqttClientImpl.java

@@ -390,7 +390,9 @@ final class MqttClientImpl implements MqttClient {
     }
 
     public void onSuccessfulReconnect() {
-        callback.onSuccessfulReconnect();
+        if (callback != null) {
+            callback.onSuccessfulReconnect();
+        }
     }