只包含动作分支和聚合的流程，执行流程报错

[fysoft2006]

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• 当前使用的UFlo的具体版本号：

2.1.3-SNAPSHOT

• 遇到的问题详细描述：

设计了一个只包括动作的流程，有分支和聚合节点，流程执行过程中报空指针异常。

• 涉及到的异常信息（完整异常信息）：

java.lang.NullPointerException: null at com.bstek.uflo.expr.impl.ExpressionContextImpl.buildProcessInstanceContext(ExpressionContextImpl.java:134) at com.bstek.uflo.expr.impl.ExpressionContextImpl.moveContextToParent(ExpressionContextImpl.java:224) at com.bstek.uflo.process.node.JoinNode.enter(JoinNode.java:48) at com.bstek.uflo.process.flow.SequenceFlowImpl.execute(SequenceFlowImpl.java:74) at com.bstek.uflo.process.node.Node.leaveNode(Node.java:96) at com.bstek.uflo.process.node.ActionNode.leave(ActionNode.java:38) at com.bstek.uflo.process.flow.SequenceFlowImpl.execute(SequenceFlowImpl.java:79) at com.bstek.uflo.process.node.ForkNode.leave(ForkNode.java:79) at com.bstek.uflo.process.flow.SequenceFlowImpl.execute(SequenceFlowImpl.java:79) at com.bstek.uflo.process.node.Node.leaveNode(Node.java:96) at com.bstek.uflo.process.node.ActionNode.leave(ActionNode.java:38) at com.bstek.uflo.process.flow.SequenceFlowImpl.execute(SequenceFlowImpl.java:79) at com.bstek.uflo.process.node.Node.leaveNode(Node.java:96) at com.bstek.uflo.process.node.StartNode.leave(StartNode.java:92) at com.bstek.uflo.command.impl.StartProcessInstanceCommand.execute(StartProcessInstanceCommand.java:85) at com.bstek.uflo.command.impl.StartProcessInstanceCommand.execute(StartProcessInstanceCommand.java:38) at com.bstek.uflo.command.impl.SpringTransactionCommandService$1.doInTransaction(SpringTransactionCommandService.java:53) at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:140) at com.bstek.uflo.command.impl.SpringTransactionCommandService.executeCommand(SpringTransactionCommandService.java:51) at com.bstek.uflo.service.impl.DefaultProcessService.startProcess(DefaultProcessService.java:162) at com.bstek.uflo.service.impl.DefaultProcessService.startProcessById(DefaultProcessService.java:138) at com.bstek.uflo.console.handler.impl.central.CentralServletHandler.startProcess(CentralServletHandler.java:189) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.bstek.uflo.console.handler.BaseServletHandler.invokeMethod(BaseServletHandler.java:36) at com.bstek.uflo.console.handler.impl.central.CentralServletHandler.execute(CentralServletHandler.java:69) at com.bstek.uflo.console.UfloServlet.service(UfloServlet.java:82) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at com.alibaba.druid.support.http.WebStatFilter.doFilter(WebStatFilter.java:123) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790) at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:748)

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