Guard against event handler unregistering themselves
Guard against event handler unregistering themselves

Closes #257

salzhrani/prosemirror,tzuhsiulin/prosemirror,mattberkowitz/prosemirror,jacwright/prosemirror,jacwright/prosemirror,mattberkowitz/prosemirror,tzuhsiulin/prosemirror,tzuhsiulin/prosemirror,adrianheine/prosemirror,salzhrani/prosemirror,adrianheine/prosemirror,kiejo/prosemirror,salzhrani/prosemirror,adrianheine/prosemirror,kiejo/prosemirror,kiejo/prosemirror