fixing NPE when processing operations

components/device-mgt/org.wso2.carbon.device.mgt.core/src/main/java/org/wso2/carbon/device/mgt/core/operation/mgt/dao/util/OperationDAOUtil.java

@@ -231,6 +231,10 @@ public class OperationDAOUtil {
                             if (rs.getBoolean("IS_LARGE_RESPONSE")) {
                                 largeResponseIDs.add(rs.getInt("OP_RES_ID"));
                             } else {
+                                if(activityStatus.getResponses() == null){
+                                    List<OperationResponse> operationResponses = new ArrayList<>();
+                                    activityStatus.setResponses(operationResponses);
+                                }
                                 activityStatus.getResponses().add(OperationDAOUtil.getOperationResponse(rs));
                             }
                         }