Activity 5.16 查看流程图报错：
java.lang.NullPointerException: null at org.activiti.image.impl.DefaultProcessDiagramGenerator.initProcessDiagramCanvas(DefaultProcessDiagramGenerator.java:729) ~[activiti-image-generator-5.16.3.jar:5.16.3] at org.activiti.image.impl.DefaultProcessDiagramGenerator.generateProcessDiagram(DefaultProcessDiagramGenerator.java:453) ~[activiti-image-generator-5.16.3.jar:5.16.3] at org.activiti.image.impl.DefaultProcessDiagramGenerator.generateDiagram(DefaultProcessDiagramGenerator.java:383) ~[activiti-image-generator-5.16.3.jar:5.16.3] at org.activiti.image.impl.DefaultProcessDiagramGenerator.generateDiagram(DefaultProcessDiagramGenerator.java:388) ~[activiti-image-generator-5.16.3.jar:5.16.3] at com.cacss.apex.bpm.core.bpe.impl.BPEClientServiceImpl.getProcessDiagram(BPEClientServiceImpl.java:475) ~[apex.bpm.core-3.3.7s-SNAPSHOT.jar:na] at com.cacss.apex.web.process.controller.UserFlowController.diagram(UserFlowController.java:918) ~[classes/:na] at com.cacss.apex.web.process.controller.UserFlowController$$FastClassByCGLIB$$6da23618.invoke(<generated>) ~[spring-core-3.2.3.RELEASE.jar:na] at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:204) ~[spring-core-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:698) ~[spring-aop-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150) ~[spring-aop-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:91) ~[spring-aop-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172) ~[spring-aop-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:631) ~[spring-aop-3.2.3.RELEASE.jar:3.2.3.RELEASE] at com.cacss.apex.web.process.controller.UserFlowController$$EnhancerByCGLIB$$e6a32712.diagram(<generated>) ~[spring-core-3.2.3.RELEASE.jar:na] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_144] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_144] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_144] at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_144] at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:219) ~[spring-web-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132) ~[spring-web-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104) ~[spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:745) ~[spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:686) ~[spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80) ~[spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:925) [spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:856) [spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:936) [spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:827) [spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) [servlet-api.jar:na] at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:812) [spring-webmvc-3.2.3.RELEASE.jar:3.2.3.RELEASE] at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) [servlet-api.jar:na] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [catalina.jar:8.5.20] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [catalina.jar:8.5.20] at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-websocket.jar:8.5.20] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [catalina.jar:8.5.20] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [catalina.jar:8.5.20] at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:198) [javamelody-core-1.53.0.jar:1.53.0] at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:176) [javamelody-core-1.53.0.jar:1.53.0] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [catalina.jar:8.5.20] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [catalina.jar:8.5.20] at org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:399) [urlrewritefilter-4.0.4.jar:4.0.4] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [catalina.jar:8.5.20] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [catalina.jar:8.5.20] at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:61) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.AdviceFilter.executeChain(AdviceFilter.java:108) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.AdviceFilter.doFilterInternal(AdviceFilter.java:137) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:125) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:66) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.AbstractShiroFilter.executeChain(AbstractShiroFilter.java:449) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.AbstractShiroFilter$1.call(AbstractShiroFilter.java:365) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.subject.support.SubjectCallable.doCall(SubjectCallable.java:90) [shiro-core-1.2.4.jar:1.2.4] at org.apache.shiro.subject.support.SubjectCallable.call(SubjectCallable.java:83) [shiro-core-1.2.4.jar:1.2.4] at org.apache.shiro.subject.support.DelegatingSubject.execute(DelegatingSubject.java:383) [shiro-core-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.AbstractShiroFilter.doFilterInternal(AbstractShiroFilter.java:362) [shiro-web-1.2.4.jar:1.2.4] at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:125) [shiro-web-1.2.4.jar:1.2.4] at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346) [spring-web-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:259) [spring-web-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [catalina.jar:8.5.20] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [catalina.jar:8.5.20] at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88) [spring-web-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-3.2.3.RELEASE.jar:3.2.3.RELEASE] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [catalina.jar:8.5.20] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [catalina.jar:8.5.20] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) [catalina.jar:8.5.20] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [catalina.jar:8.5.20] at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) [catalina.jar:8.5.20] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [catalina.jar:8.5.20] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) [catalina.jar:8.5.20] at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650) [catalina.jar:8.5.20] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [catalina.jar:8.5.20] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [catalina.jar:8.5.20] at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) [tomcat-coyote.jar:8.5.20] at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-coyote.jar:8.5.20] at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) [tomcat-coyote.jar:8.5.20] at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1457) [tomcat-coyote.jar:8.5.20] at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-coyote.jar:8.5.20] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_144] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_144] at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-util.jar:8.5.20] at java.lang.Thread.run(Thread.java:748) [na:1.8.0_144]
原因：这是因为流程图定义中，某个或多个元素未设置ID,如下图所示：
