fixing https://github.com/wso2/product-iots/issues/1136 : DeviceDetailsRetrieverTask attempts to add operations to a device that has been removed

components/device-mgt/org.wso2.carbon.device.mgt.core/src/main/java/org/wso2/carbon/device/mgt/core/task/impl/DeviceTaskManagerImpl.java

@@ -92,7 +92,7 @@ public class DeviceTaskManagerImpl implements DeviceTaskManager {
             operations = this.getValidOperationNames(); //list operations for each device type
             devices = deviceManagementProviderService.getAllDevices(deviceType, false);//list devices for each type
             if (!devices.isEmpty()) {
-                if (operations != null) {
+                if (operations != null && DeviceManagerUtil.getValidDeviceIdentifiers(devices).size() != 0) {
                     for (String str : operations) {
                         CommandOperation operation = new CommandOperation();
                         operation.setEnabled(true);

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

@@ -247,6 +247,7 @@ public final class DeviceManagerUtil {
                 switch (device.getEnrolmentInfo().getStatus()) {
                     case BLOCKED:
                     case REMOVED:
+                        break;
                     case SUSPENDED:
                         break;
                     default: