Sets contain entity type &1, which no longer exists

What causes this issue?

The entity &V2& of entity type &V3& contains sets of the entitiy type
&V1&. However, the entity type &V1& is no longer in your data model. It
has probably been deleted. The set data of entity type &V1& is therefore
invalid and will be deleted next time you save.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Save the entity &V2& of entity type &V3& to clean up your invalid set
data.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.