At the moment, the default background colour for NULL fields is a very strong red. It makes things seem like there's an error, when there's not:

Maybe #ffd0cd instead?

Even lighter might be better too (unsure). It depends on how the colour displays across a wide selection of monitors...

[side question - how does a person reset an existing field with a value to NULL? I just tried and didn't find a reliable method. 😦 In the edit dialog for a field, changing to "binary" then clicking "clear" and exiting works initially, but then the table goes white/non-NULL again after changing to the Database Structure tab and back again (sounds like a bug really) 😉]