Improvement: disable stacktrace tab for closed workflows, or telling why it's empty

[longquanzheng]

The stackrtrace is always empty for closed workflows. This is because stacktrace relies on workflow being blocked on certain code and processed a built-in query request. If a workflow is closed, there is no way to get such a stacktrace.

[just-at-uber]

Perhaps we can show a more descriptive message on the stacktrace tab explaining why a user can't get the current stacktrace and remove the refresh button