Issue

Not able to import process into web-designer if process contains push-down subprocess.
Steps to Reproduce:
1. Create process in JBDS which contains embedded subprocess.
2. Push-down subprocess,and save process definition.
3. Now try to import process into web-designer, it will fail with NPE

ERROR [stderr] (http-/127.0.0.1:8080-5) java.lang.NullPointerException
ERROR [stderr] (http-/127.0.0.1:8080-5)     at org.jbpm.designer.bpmn2.impl.Bpmn2JsonMarshaller.marshallNode(Bpmn2JsonMarshaller.java:2195)
ERROR [stderr] (http-/127.0.0.1:8080-5)     at org.jbpm.designer.bpmn2.impl.Bpmn2JsonMarshaller.marshallStartEvent(Bpmn2JsonMarshaller.java:884)
ERROR [stderr] (http-/127.0.0.1:8080-5)     at org.jbpm.designer.bpmn2.impl.Bpmn2JsonMarshaller.marshallFlowElement(Bpmn2JsonMarshaller.java:836)
ERROR [stderr] (http-/127.0.0.1:8080-5)     at org.jbpm.designer.bpmn2.impl.Bpmn2JsonMarshaller.marshallSubProcess(Bpmn2JsonMarshaller.java:2703)
ERROR [stderr] (http-/127.0.0.1:8080-5)     at org.jbpm.designer.bpmn2.impl.Bpmn2JsonMarshaller.marshallFlowElement(Bpmn2JsonMarshaller.java:865)
...