App does not allow NULL value on Value Relation widget

[raherin]

Description

In QGIS there is an option to enable 'Allow null values' on the Value Relation widget settings. This will create an option to choose 'no selection' in the drop down menu on the form in QGIS. However, in Mergin Maps, this option is not available when using the widget. This can be problematic if the user in the field needs to change a value on an already saved feature from any given value to a null value, currently there is no way to do this in the app.

Environment
Production

Application (+ app version, build, operating system)
Mergin Maps 2025.7.0
Android 16

Steps to reproduce

• Create a project in QGIS with:
  • a spatial layer that has a field for recording the value relation
  • a non-spatial layer with a unique key field and a value field
• Fill in the non-spatial layer with unique key values like uuid() and values for the options field
• Set up the spatial layer form with a Value Relation widget to record the value from the non-spatial field, make sure to enable the 'Allow null values' option

• Save the project and upload to Mergin Maps
• Download the project in the app
• Add a new feature to the spatial layer and select the value relation widget dropdown menu, there is no option for NULL value

Actual results

It is not possible to select a NULL value in the Mergin Maps app

Expected results

In QGIS, the drop-down menu for the value relation includes an option to choose 'no selection'