The function ui_draw_popover_back has a "block" argument, that is
checked for null, but then has an "else" that would require it and
cause a null pointer dereference. This is an internal function that has
one caller, in response to a condition on that same block's flag.
Therefore this PR removes this function's check for null and the else
code as this condition is not possible.
Pull Request: https://projects.blender.org/blender/blender/pulls/119092
a1f92f5820