ContextMenu: Clean up target and spell checker only after callback

Cleaning up spell checker before executing callback code that relied on
it being still there didn't go very well. Besides, as soon as spell
checker is reset to null, menu items for each suggestion is destroyed,
taking down their JavaScript context with them, so callback closures
bound to (created in) such context would not be able to run properly
anymore: object ids like `root` would suddenly turn into null, context
properties would either become undefined or raise exception when
accessed etc.