Fixing NPE of EMM-1715

components/policy-mgt/org.wso2.carbon.policy.mgt.core/src/main/java/org/wso2/carbon/policy/mgt/core/mgt/impl/MonitoringManagerImpl.java

@@ -380,8 +380,11 @@ public class MonitoringManagerImpl implements MonitoringManager {
 
         List<String> deviceTypes = new ArrayList<>();
         try {
+		//when shutdown, it sets DeviceManagementService to null, therefore need to have a null check
+		if (PolicyManagementDataHolder.getInstance().getDeviceManagementService() != null) {
             deviceTypes =
                     PolicyManagementDataHolder.getInstance().getDeviceManagementService().getAvailableDeviceTypes();
+		}
         } catch (DeviceManagementException e) {
             throw new PolicyComplianceException("Error occurred while getting the device types.", e);
         }