Small fix for Volume Editor.cs for newer Unity versions. (tested on 2022.3.33f1 LTS)

[MrOnsku]

When you entered edit mode for a volume, you would get a lot of errors and the editor would be unusable. I have fixed it so that this does not happen. The only things that are not working are the box operations (box paint, box subtract, etc.) https://paste.ofcode.org/32ttbfZDaKbeR5XbS4Dw5tp