I modelled a ServiceTask with multiinstance behaviour (asynchronous and sequential). The task implementation extends AbstractBpmnActivityBehavior.
Furthermore I configured the engine to use the "CdiEventSupportBpmnParseListener" to use the cdi event mechanism.

If there is more than one execution for the multiinstance activity the leave()-method fails with a NullPointerException in the CdiEventListener because there is no execution context.
org.camunda.bpm.engine.cdi.impl.event.CdiEventListener:
protected BusinessProcessEvent createEvent(DelegateExecution execution) {
ProcessDefinitionEntity processDefinition = Context.getExecutionContext().getProcessDefinition();

The createEvent(DelegateTask task)-method in the same class checks first if there is a ExecutionContext. After changing the createEvent()-method for an DelegateExecution to do the same it works or at least there is no exception.

I attached a test project to reproduce the exception. To run the junit test include the jar in \lib directory in classpath to get the cdi container running.