Cannot update a sent push notification sometimes

[david-venhoff]

Describe the Bug

Sometimes an error message shows up that a required field is not set, even though the mode is set to fallback to default language:

Steps to Reproduce

1. Create a push notification in a region that has a root language which has an id that is bigger than the id of another language of that region
2. send it
3. try to update it
4. notice the error

Expected Behavior

The pn can be updated normally

Actual Behavior

The pn cannot be updated

Additional Information

The problem seems to be that the code assumes that the default language of the region is the first language of the formset, while the actual first language of the formset is the language with the smallest id