ui_lists shall be NULLed in BKE_area_region_copy

Otherwise it'll be nasty crashes when, say, adding and removing
screens with lists visible on the screen.

Thanks Ton for assisting looking into this issue :)

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53638 954f8c5b-7b00-dc11-b283-0030488c597c