Fix regression in PathEventProxy.disconnect.
Fix regression in PathEventProxy.disconnect.

stencila/substance,TypesetIO/substance,jacwright/substance,substance/substance,podviaznikov/substance,substance/substance,michael/substance-1,TypesetIO/substance,abhaga/substance,andene/substance,podviaznikov/substance,jacwright/substance,michael/substance-1,andene/substance,abhaga/substance