Fixes T10346. You finally wrung a clue out of the reporter and I think I figured this out.
Here's the bug:
- Create a project with a workboard and subprojects/milestones.
- Create a new project, import columns from the first project.
- We incorrectly import empty columns for the subprojects/milestons.
Instead, skip proxy columns during import.
Also, allow "hide column" to continue on missing fields, so columns with no name can be hidden.