创建子流程

使用JBPM创建子流程时出错
org.jbpm.JbpmException: can't create a process instance when processDefinition is null
    at org.jbpm.graph.exe.ProcessInstance.<init></init>(ProcessInstance.java:109)
    at org.jbpm.graph.exe.ProcessInstance.<init></init>(ProcessInstance.java:91)
    at org.jbpm.graph.exe.Token.createSubProcessInstance(Token.java:612)
    at org.jbpm.graph.node.ProcessState.execute(ProcessState.java:157)
    at org.jbpm.graph.def.Node.enter(Node.java:318)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at org.hibernate.proxy.pojo.cglib.CGLIBLazyInitializer.invoke(CGLIBLazyInitializer.java:147)
    at org.jbpm.graph.def.Node$$EnhancerByCGLIB$$1afd3471.enter(<generated></generated>)
    at org.jbpm.graph.def.Transition.take(Transition.java:151)
    at org.jbpm.graph.def.Node.leave(Node.java:393)
    at org.jbpm.graph.node.TaskNode.leave(TaskNode.java:209)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at org.hibernate.proxy.pojo.cglib.CGLIBLazyInitializer.invoke(CGLIBLazyInitializer.java:147)
    at org.jbpm.graph.node.TaskNode$$EnhancerByCGLIB$$55432abb.leave(<generated></generated>)
    at org.jbpm.graph.exe.Token.signal(Token.java:194)
    at org.jbpm.graph.exe.Token.signal(Token.java:139)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at org.hibernate.proxy.pojo.cglib.CGLIBLazyInitializer.invoke(CGLIBLazyInitializer.java:147)
    at org.jbpm.graph.exe.Token$$EnhancerByCGLIB$$c78bdb05.signal(<generated></generated>)
    at org.jbpm.taskmgmt.exe.TaskInstance.end(TaskInstance.java:475)
    at org.jbpm.taskmgmt.exe.TaskInstance.end(TaskInstance.java:389)
    at com.linkage.olcom.jbpm.task.TaskOperation.completeTask(TaskOperation.java:166)
    at com.linkage.olcom.jbpm.process.createproject.ProcessTest.testProcess(ProcessTest.java:84)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:585)
    at junit.framework.TestCase.runTest(TestCase.java:154)
    at junit.framework.TestCase.runBare(TestCase.java:127)
    at junit.framework.TestResult$1.protect(TestResult.java:106)
    at junit.framework.TestResult.runProtected(TestResult.java:124)
    at junit.framework.TestResult.run(TestResult.java:109)
    at junit.framework.TestCase.run(TestCase.java:118)
    at junit.framework.TestSuite.runTest(TestSuite.java:208)
    at junit.framework.TestSuite.run(TestSuite.java:203)
    at org.eclipse.jdt.internal.junit.runner.junit3.JUnit3TestReference.run(JUnit3TestReference.java:128)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:460)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:673)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:386)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)

数据库中已保存了父流程和子流程，不知道是什么问题，有没有高手可以指教一下？