switching to the sidebar destroys the control currently active,
just post the event to happen on the next event loop

Change-Id: I3e9b629821fa43d8f29e419e72f66ed46d72fdbb
Reviewed-on: https://gerrit.libreoffice.org/65543
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>