Process without start event can be deployed but not started

Component

https://github.com/camunda/camunda-bpm-platform/tree/master/engine

Steps to reproduce

• Download the Camunda full distribution, e.g. for tomcat, and start it (or any other way to start a process engine)
• Create a BPMN model without a start event, e.g. WithoutStart.bpmn
• Deploy the process

Observed behavior

• The process deploys successfully
• Start of the process instance fails with the following exception:

  25-Jun-2020 11:45:28.420 SEVERE [http-nio-8080-exec-2] org.camunda.commons.logging.BaseLogger.logError ENGINE-16004 Exception while closing command context: No startFormHandler defined in process 'ProcWithoutStart:1:53f3e7ad-b625-11ea-88fd-acde48001122': startFormHandler is null
  	org.camunda.bpm.engine.exception.NullValueException: No startFormHandler defined in process 'ProcWithoutStart:1:53f3e7ad-b625-11ea-88fd-acde48001122': startFormHandler is null
  		at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
  		at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
  		at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
  		at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
  		at org.camunda.bpm.engine.impl.util.EnsureUtil.generateException(EnsureUtil.java:394)
  		at org.camunda.bpm.engine.impl.util.EnsureUtil.ensureNotNull(EnsureUtil.java:55)
  		at org.camunda.bpm.engine.impl.util.EnsureUtil.ensureNotNull(EnsureUtil.java:50)
  		at org.camunda.bpm.engine.impl.cmd.GetStartFormCmd.execute(GetStartFormCmd.java:57)
  		at org.camunda.bpm.engine.impl.cmd.GetStartFormCmd.execute(GetStartFormCmd.java:37)
  		at org.camunda.bpm.engine.impl.interceptor.CommandExecutorImpl.execute(CommandExecutorImpl.java:28)
  		at org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:110)
  		at org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor.execute(ProcessApplicationContextInterceptor.java:70)
  		at org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:33)
  		at org.camunda.bpm.engine.impl.FormServiceImpl.getStartFormData(FormServiceImpl.java:63)
  		at org.camunda.bpm.engine.rest.sub.repository.impl.ProcessDefinitionResourceImpl.getStartForm(ProcessDefinitionResourceImpl.java:316)
  		at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  		at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  		at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  		at java.base/java.lang.reflect.Method.invoke(Method.java:566)
  		at org.jboss.resteasy.core.MethodInjectorImpl.invoke(MethodInjectorImpl.java:138)
  		at org.jboss.resteasy.core.ResourceMethodInvoker.internalInvokeOnTarget(ResourceMethodInvoker.java:526)
  		at org.jboss.resteasy.core.ResourceMethodInvoker.invokeOnTargetAfterFilter(ResourceMethodInvoker.java:415)
  		at org.jboss.resteasy.core.ResourceMethodInvoker.lambda$invokeOnTarget$0(ResourceMethodInvoker.java:376)
  		at org.jboss.resteasy.core.interception.PreMatchContainerRequestContext.filter(PreMatchContainerRequestContext.java:356)
  		at org.jboss.resteasy.core.ResourceMethodInvoker.invokeOnTarget(ResourceMethodInvoker.java:378)
  		at org.jboss.resteasy.core.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:347)
  		at org.jboss.resteasy.core.ResourceLocatorInvoker.invokeOnTargetObject(ResourceLocatorInvoker.java:150)
  		at org.jboss.resteasy.core.ResourceLocatorInvoker.invoke(ResourceLocatorInvoker.java:110)
  		at org.jboss.resteasy.core.ResourceLocatorInvoker.invokeOnTargetObject(ResourceLocatorInvoker.java:141)
  		at org.jboss.resteasy.core.ResourceLocatorInvoker.invoke(ResourceLocatorInvoker.java:104)
  		at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:440)
  		at org.jboss.resteasy.core.SynchronousDispatcher.lambda$invoke$4(SynchronousDispatcher.java:229)
  		at org.jboss.resteasy.core.SynchronousDispatcher.lambda$preprocess$0(SynchronousDispatcher.java:135)
  		at org.jboss.resteasy.core.interception.PreMatchContainerRequestContext.filter(PreMatchContainerRequestContext.java:356)
  		at org.jboss.resteasy.core.SynchronousDispatcher.preprocess(SynchronousDispatcher.java:138)
  		at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:215)
  		at org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.service(ServletContainerDispatcher.java:227)
  		at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:56)
  		at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:51)
  		at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
  		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:53)
  		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
  		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
  		at org.camunda.bpm.engine.rest.filter.CacheControlFilter.doFilter(CacheControlFilter.java:45)
  		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
  		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
  		at org.camunda.bpm.engine.rest.filter.EmptyBodyFilter.doFilter(EmptyBodyFilter.java:101)
  		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
  		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
  		at org.camunda.bpm.webapp.impl.security.filter.headersec.HttpHeaderSecurityFilter.doFilter(HttpHeaderSecurityFilter.java:87)
  		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
  		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
  		at org.camunda.bpm.webapp.impl.security.filter.CsrfPreventionFilter.doFilter(CsrfPreventionFilter.java:175)
  		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
  		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
  		at org.camunda.bpm.webapp.impl.security.filter.SecurityFilter.doFilterSecure(SecurityFilter.java:73)
  		at org.camunda.bpm.webapp.impl.security.filter.SecurityFilter.doFilter(SecurityFilter.java:57)
  		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
  		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
  		at org.camunda.bpm.webapp.impl.security.auth.AuthenticationFilter$1.execute(AuthenticationFilter.java:62)
  		at org.camunda.bpm.webapp.impl.security.auth.AuthenticationFilter$1.execute(AuthenticationFilter.java:60)
  		at org.camunda.bpm.webapp.impl.security.SecurityActions.runWithAuthentications(SecurityActions.java:44)
  		at org.camunda.bpm.webapp.impl.security.auth.AuthenticationFilter.doFilter(AuthenticationFilter.java:60)
  		at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
  		at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
  		at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
  		at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
  		at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
  		at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
  		at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
  		at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:688)
  		at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
  		at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
  		at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)
  		at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
  		at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
  		at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1594)
  		at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
  		at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
  		at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
  		at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
  		at java.base/java.lang.Thread.run(Thread.java:834)
  

Expected behavior

• Deployment should fail with an error message explaining the reason