ContextMenu: Fix callbacks
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.
Wrap callback in a try-catch
Just in case, so that we don't have to trust all the actions to be well-behaved, which might be even more dangerous if we were to allow the menu to be extended with 3rd-party items.