TypeError: argument of type 'NoneType' is not iterable

An error has occurred while executing Python code:
TypeError: argument of type 'NoneType' is not iterable
Traceback (most recent call last):
File "C:\Users/USER/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\Mergin\plugin.py", line 308, in current_project_sync
self.manager.project_status(self.mergin_proj_dir)
File "C:\Users/USER/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\Mergin\projects_manager.py", line 163, in project_status
validation_results = validator.run_checks()
File "C:\Users/USER/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\Mergin\validation.py", line 90, in run_checks
self.check_value_relation()
File "C:\Users/USER/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\Mergin\validation.py", line 273, in check_value_relation
self._check_primary_keys(child_layer, [idx])
File "C:\Users/USER/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\Mergin\validation.py", line 285, in _check_primary_keys
if layer_fields[i].name() in keys:
TypeError: argument of type 'NoneType' is not iterable

Start in validation.py at run_checks(), check_value_relation(), and _check_primary_keys(), following the traceback from projects_manager.py:163 and plugin.py:308. Reproduce the current project-status check and determine why keys is None; done means current_project_sync completes without this TypeError and the validation result remains available.