[Help]"attempt to call a nil value" - No further info

Hello everyone. I recently switched to Sethik's UI, and i started getting some weird "attemp to call a nil value" error msg's.. I like the UI alot and would hate to switch away again.But there is no further info about the value, anyone know this error and that have any tips on it?Regards, Ensey.(P.S. It pops up on Call Pet and Dismiss Pet)
I got the same error with FuBar (specially ClockFu & PetInfoFu -addons). "Attempt to call a nil value"-error appeared every now and then but usually always when I hovered mouse above "Addon Preferences"-button on the minimap. Disabling those FuBar-plugins helped, but I still wanted to use them so I rebuilt my UI.Maybe that helps for you too?, run full Wowrepair (delete Cache & Addons) or delete those folders manually. Then reinstall all your addons.