When I try to open a villagers GUI when it does not have an workstation occupied, the GUI closes instantly after opening. All you see is a flash of the GUI.

How to reproduce:
Spawn a villagers.
Give the villager a workstation
Trade with the villager, such that the profession is locked.
Then there are 2 options:
1. Just remove the workstations.
2. Make sure a new (second) villager occupies the workstation of the other workstation.

The villager has to realize that his workstation is gone. Therefore, make sure to check this within working hours. Also, the villager probably needs a few seconds after the start of the working hours to realize that the workstation is gone.

Now, the first villager does have a profession, but not a workstation. When you try to open the GUI, it closes instantly after opening. This happens when the workstation is removed, or when the workstation is occupied by another villager. I've seen this happen with lecterns (librarians) and with composters (farmers).

When you place a workstation suiting the profession, and the villager occupies this workstation, the GUI behaves as normal.