Menu entry dispatchers may be cleared (after they've been initially
set) when an extension comes with its own menu entries. Activating
the entry sets the dispatcher again so that dispatching a command
can proceed

Change-Id: I3909fb5eb6e6e2fa7db0418aca17009e60f01372
Reviewed-on: https://gerrit.libreoffice.org/62180
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>