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I've made a field in a layer without NULL and connected it by relation to a fixed list of values from another layer by using Edit widget:Value Relation. Then I checked "Use completer" option in that widet to use an edit line in this field(you can see it in the picture attached).

When I make a new object in that layer and complete a value by editing it, I always get invalid field error just like I would still have null in that field. I can't put any objects in that layer.
When I uncheck "Use completer" in edit widget, than I have to use a combobox in that field and this way gives no error.

So using field properties: without NULL, Edit widget:Value Relation with Use completer checked - gives error when creating any object in that layer.