[Fix-8316][Master] Stop dependent node error (#8323)

* fix bug_8316

* fix bug_8316

* fix bug_8316

dolphinscheduler-server/src/main/java/org/apache/dolphinscheduler/server/master/processor/queue/TaskResponsePersistThread.java

@@ -74,7 +74,7 @@ public class TaskResponsePersistThread implements Runnable {
                     logger.error("persist meta error, task id:{}, instance id:{}", event.getTaskInstanceId(), event.getProcessInstanceId());
                 }
             } catch (Exception e) {
-                logger.error("persist error, task id:{}, instance id:{}", event.getTaskInstanceId(), event.getProcessInstanceId(), e);
+                logger.error("persist error, task id:{}, instance id:{}, error: {}", event.getTaskInstanceId(), event.getProcessInstanceId(), e);
             } finally {
                 this.events.remove(event);
             }
@@ -158,8 +158,12 @@ public class TaskResponsePersistThread implements Runnable {
                         logger.debug("ACTION_STOP: task instance id:{}, process instance id:{}", taskResponseEvent.getTaskInstanceId(), taskResponseEvent.getProcessInstanceId());
                     }
                 }
-                TaskKillAckCommand taskKillAckCommand = new TaskKillAckCommand(ExecutionStatus.SUCCESS.getCode(), taskResponseEvent.getTaskInstanceId());
-                channel.writeAndFlush(taskKillAckCommand.convert2Command());
+
+                if (channel != null) {
+                    TaskKillAckCommand taskKillAckCommand = new TaskKillAckCommand(ExecutionStatus.SUCCESS.getCode(), taskResponseEvent.getTaskInstanceId());
+                    channel.writeAndFlush(taskKillAckCommand.convert2Command());
+                }
+
                 break;
             default:
                 throw new IllegalArgumentException("invalid event type : " + event);