[FIX-5908][MasterServer] When executing an compensation task, the execution thread would have a NPE (#5909)

* fix the npe in MasterExec

* fix the compile error

dolphinscheduler-server/src/main/java/org/apache/dolphinscheduler/server/master/runner/MasterExecThread.java

@@ -46,6 +46,7 @@ import org.apache.dolphinscheduler.common.utils.JSONUtils;
 import org.apache.dolphinscheduler.common.utils.OSUtils;
 import org.apache.dolphinscheduler.common.utils.ParameterUtils;
 import org.apache.dolphinscheduler.common.utils.StringUtils;
+import org.apache.dolphinscheduler.dao.entity.ProcessDefinition;
 import org.apache.dolphinscheduler.dao.entity.ProcessInstance;
 import org.apache.dolphinscheduler.dao.entity.ProjectUser;
 import org.apache.dolphinscheduler.dao.entity.Schedule;
@@ -976,7 +977,9 @@ public class MasterExecThread implements Runnable {
                         task.getName(), task.getId(), task.getState());
                 // node success , post node submit
                 if (task.getState() == ExecutionStatus.SUCCESS) {
+                    ProcessDefinition relatedProcessDefinition = processInstance.getProcessDefinition();
                     processInstance = processService.findProcessInstanceById(processInstance.getId());
+                    processInstance.setProcessDefinition(relatedProcessDefinition);
                     processInstance.setVarPool(task.getVarPool());
                     processService.updateProcessInstance(processInstance);
                     completeTaskList.put(task.getName(), task);