UIMA AS aggregate not handling mis-configured FlowController

If a flow controller is misconfigured with a wrong AE key, the Uima AS aggregate is not handling an invalid step object (null) returned from the flow controller's next() method. In executeFlowStep() the aggregate code checks for three types of objects: SimpleStep, ParallelStep, and FinalStep. There is no default section to handle invalid type of Step object. This leads to a lost CAS and may eventually cause a hang due to exhausted CAS pool. The current code never transitions a CAS to the finalStep and subsequent release causing eventual hang.