Hi there,
thanks for reporting! Seems it's a very old bug, but was never
recognized/mentioned! I just fixed that for the next Reminderfox release
(2.1.6) which I hope we can ship to users very soon.
With your current version there is a little workaround you can do:
If you like to delete a "not used category" always first delete the
'null' category and then the one in mind.
Hope it helps for the time being .. or just ignore it, it shouldn't harm
you that much =-O.
Thanks again
Günter