Workflow gets stuck with wt.workflow.definer.InvalidEventException: null event error in Windchill PDMLink

• Workflow gets stuck
• With severe entry in WfPropagationQueue queue
• Error in Background Method log:

ERROR : wt.queue.ProcessingQueue.WfPropagationQueue wcadmin - Queue entry number: xxxxxx
java.lang.reflect.InvocationTargetException
    at sun.reflect.GeneratedMethodAccessor101.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
    at java.lang.reflect.Method.invoke(Method.java:611)
    at wt.queue.QueueEntry.execute(QueueEntry.java:231)
    at wt.queue.ProcessingQueue.execEntry(ProcessingQueue.java:287)
    at wt.queue.ProcessingQueue.execEntries(ProcessingQueue.java:877)
    at wt.queue.PollingQueueThread.run(PollingQueueThread.java:109)
Caused by: wt.workflow.WfException: wt.util.WTException: wt.workflow.definer.InvalidEventException: null event
Nested exception is: wt.workflow.definer.InvalidEventException: null event
Nested exception is: wt.util.WTException: wt.workflow.definer.InvalidEventException: null event
Nested exception is: wt.workflow.definer.InvalidEventException: null event
    at wt.workflow.engine.StandardWfEngineService.propagateEvent(StandardWfEngineService.java:880)
    ... 7 more
Caused by: wt.util.WTException: wt.workflow.definer.InvalidEventException: null event
Nested exception is: wt.workflow.definer.InvalidEventException: null event
    at wt.workflow.definer.WfExpression.executeRouter(WfExpression.java:1341)
    at wt.workflow.definer.WfExpression.execute(WfExpression.java:416)
    at wt.workflow.engine.WfConnector.evaluateRouterExpression(WfConnector.java:586)
    at wt.workflow.engine.WfConnector.fire(WfConnector.java:350)
    at wt.workflow.engine.WfConnector.fireInput(WfConnector.java:314)
    at wt.workflow.engine.ActivityConnectorLink.receiveEvent(ActivityConnectorLink.java:143)
    at wt.workflow.engine.StandardWfEngineService.propagateEvent(StandardWfEngineService.java:857)
    ... 7 more
Caused by: wt.workflow.definer.InvalidEventException: null event
    at wt.workflow.expr.WfExpression10325578557.execute_ROUTER_EXPRESSION_(WfExpressionYYYYYYY.java:164)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:60)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
    at java.lang.reflect.Method.invoke(Method.java:611)
    at wt.workflow.definer.WfExpression.invokeByReflectionWithTransaction(WfExpression.java:1204)
    at wt.workflow.definer.WfExpression.executeRouter(WfExpression.java:1322)
    ... 13 more