During editing attributes of any entity, you can select “Not Null” at the Attributes tab. There is no problem with changing “Not Null” status here.
After selecting attribute to edition (select attribute and click Edit), when you want to mark “Not Null” checkbox, you have to click it 2 times. After first click your mark disappears. All next clicks behave properly.
This situation is reversible - when you have “Not Null” set, you can uncheck it on Attribute tab by one click, but after opening attribute to edition, you have to click it two times.
PS. If I have any other comments to GUI, I’ll post them in this thread. That’s why the topic is so long