I've had this bug show up recently since new patch just launched. It shows up every time after a load screen, whether logging in for first time or simply zoning to a new location.
Code:
1x nibRealUI\Modules\ActionBars.lua:108: attempt to perform arithmetic on local 'numButtons' (a nil value)
nibRealUI\Modules\ActionBars.lua:108: in function 'ApplyABSettings'
nibRealUI\Modules\ActionBars.lua:502: in function '?'
...nit\libs\CallbackHandler-1.0\CallbackHandler-1.0-6.lua:147: in function <...nit\libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:147>
[string "safecall Dispatcher[1]"]:4: in function <[string "safecall Dispatcher[1]"]:4>
[C]:: ?
[string "safecall Dispatcher[1]"]:13: in function '?'
...nit\libs\CallbackHandler-1.0\CallbackHandler-1.0-6.lua:92: in function 'Fire'
...\DataStore\libs\AceEvent-3.0\AceEvent-3.0-3.lua:120: in function <...\DataStore\libs\AceEvent-3.0\AceEvent-3.0.lua:119>
Shortly after posting this I noticed my action bar were misaligned, I tried to tinker with the setting to see what's wrong and it appears the bug is affecting RealUIs control of Bartender because changing any of the setting has no effect on Bartender and only produces more errors.