If you do not have ExtCal2 installed, you get this error. I might have to introduce a first check so it the ExtCal configuration is disabled if it is not installed.
This should not cause any troubles what so ever, if you do not have ExtCal2 installed. IT does _not_ depend on it, but if you have it installed, it will integrate.
In com_mls_events 0.1.1, you will not see the error in the configuration page anymore.