Generic Villager GUI Issue

You can only open Generic Villager GUI "Nitwit" once.

What I think should happen was that the GUI should be able to open multiple times.

You can only open it once.

*BUG HAPPENS WHEN*
1. Summon Generic Villager by egg or /summon Villager ~ ~ ~

{Profession:5}

2. Open its GUI "Nitwit"
3. YOU NOW CAN'T OPEN IT AGAIN.