When I tested , I did not have this error. But you can try something else. Install brightness back, and see if this issue back. If yes, try to play with tweak BB-1-2 and 1-1.. if still issue, try to reinstall lockscreen package (see pdf), If still issue, maybe you system needed reflash

P.S. when you apply brightness applet it will also automatically apply option BB-1-1 for hacky-brightness...