Github user revans2 commented on the issue:
https://github.com/apache/storm/pull/2816
@danny0405 @agresch this should not happen under normal conditions. If it
is happening under normal conditions we need to fix some things. The approved
workers holds the list of workers that the supervisor believes it is currently
running. The only times that it would become null would be when there is an
issue with the local state, like it was wiped clean too quickly by an over
aggressive script meant to fix a broken node, or by a test framework that
possibly got the timing wrong.
If this is happening regularly outside of odd situations like I said, then
there may be a race condition that we missed and we need to look into it more
deeply.
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