Web UI triggers unsupported operand type(s) for /: 'WindowsPath' and 'NoneType' if reference beatmap does not have a background specified in the .osu file

[piee-kun]

If using web ui, choosing an .osu file that does not contain a background image, validate_paths() errors with

unsupported operand type(s) for /: 'WindowsPath' and 'NoneType'

specifying background allows the program to work

[troknastya]

same problem. Error during path validation: unsupported operand type(s) for /: 'WindowsPath' and 'NoneType'