Misc/NEWS.d/next/Library/2019-03-20-15-13-18.bpo-36366.n0eav_.rst

Calling ``stop()`` on an unstarted or stopped :func:`unittest.mock.patch`
object will now return `None` instead of raising :exc:`RuntimeError`,
making the method idempotent.
Patch by Karthikeyan Singaravelan.