date_end_effective integrity error

[Mustafa Rawi]

When creating a new contract, I get:
Integrity Error

The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: date_end_effective - date.end.effective]

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Update:
Tested on a fresh database with demo data. Modifying demo employees raises errors. Adding new employees, everything works normally.

Closing this bug unless you want to check it out.