WMF ran into this after their update. Here's the setup:
- When you enable Spaces, we leave all existing objects set to null, which means "these belong to the default space". This is so we don't have to go update a trillion objects.
- New objects get set to the default space explicitly (PHID-SPCE-...) but older ones stay with null.
- If you edit an older object (like a task) from the time before Spaces, and the form doesn't have a Visbility/Spaces control, we would incorrectly poplate the value with null when the effective value should be the default space PHID.
- This caused a "You must choose a space." error in the UI.
Instead, populate the control with the effective value instead of the literal database value. This makes the edit go through cleanly.
Also add a note about this for future-me.