Fixing NPE thrown while shutting down the MDM server

components/device-mgt/org.wso2.carbon.device.mgt.mobile.impl/src/main/java/org/wso2/carbon/device/mgt/mobile/internal/MobileDeviceManagementServiceComponent.java

@@ -102,10 +102,15 @@ public class MobileDeviceManagementServiceComponent {
             log.debug("De-activating Mobile Device Management Service Component");
         }
         try {
+            if (androidServiceRegRef != null) {
                 androidServiceRegRef.unregister();
+            }
+            if (iOSServiceRegRef != null) {
                 iOSServiceRegRef.unregister();
+            }
+            if (windowsServiceRegRef != null) {
                 windowsServiceRegRef.unregister();
-            serverStartupObserverRef.unregister();
+            }
 
 			if (log.isDebugEnabled()) {
 				log.debug(