NPE exception happens if it throws exception in finishBundle during job shutdown

Currently, if it throws exceptions in finishBundle during job shutdown, NPE exception may happen if time-based finish bundle is scheduled. It caused the actual exception isn't propagate. This makes users very difficult to trouble shot the problem.

See http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/PyFlink-called-already-closed-and-NullPointerException-td42997.html for more details.