Added null check to resolve wso2/product-iots#1367

components/device-mgt/org.wso2.carbon.device.mgt.core/src/main/java/org/wso2/carbon/device/mgt/core/DeviceManagementPluginRepository.java

@@ -350,7 +350,9 @@ public class DeviceManagementPluginRepository implements DeviceManagerStartupLis
         if (deviceStatusTaskPluginConfig != null && deviceStatusTaskPluginConfig.isRequireStatusMonitoring()) {
             try {
                 DeviceManagementDataHolder.getInstance().removeDeviceStatusTaskPluginConfig(deviceType);
+                if (deviceStatusTaskManagerService != null) {
                     deviceStatusTaskManagerService.stopTask(deviceType, deviceStatusTaskPluginConfig);
+                }
             } catch (DeviceStatusTaskException e) {
                 throw new DeviceManagementException("Error occurred while stopping Device Status task service for '" +
                         deviceManagementService.getType() + "'", e);