Is there a “safety-net” mechanism at national level allowing a validation of my classical European patent in case my request for unitary effect is rejected?

Yes, most Member States have implemented a so-called safety net which enables the patent holder to validate a European patent where a request for unitary effect has been rejected either by the EPO or the Unified Patent Court, or where the unitary effect has been revoked by the Unified Patent Court at the time when the time periods for validating the European patent have already expired.

An overview of the relevant national laws and practices implementing this mechanism is available in the publication

"National measures relating to the Unitary Patent", 1st edition / September 2022, part II “Safety net” where relevant information can be retrieved on a country-by-country basis.