Unnecessary popup "Save edits to Manhole?" in specific cases

[leendertvanwolfswinkel]

I get several "Do you want to save changes to the layer Manhole" popups (I think four times, between data validation and saving to spatialite).

This also happens if I toggle editing on for the Pipe layer and then off again, even when I haven't edited anything.

How to reproduce the issue:

• Load the following spatialite: Hekerbeek na maatregelen Gebiedsbreed T25.zip
• Use Identify tool to open attribute form for Pipe with ID 18891
• Start editing
• Stop editing

Comment from @ldebek : Issue is not related with validation. It seems that there is an issue with handling NULL values in the custom forms comboboxes.

Originally posted by @leendertvanwolfswinkel in https://github.com/nens/threedi-schematisation-editor/issues/70#issuecomment-1265163738

[ldebek]

@leendertvanwolfswinkel Fix is on master branch now.

[leendertvanwolfswinkel]

Tested