When updating rows using the CDS Current Environment Connection lookup values must be proceeded with the entity name for the reference. For example;

But what if the ID is empty? In that case an error wil occur because the reference is null.

There is a (really nasty) solution to get around this, using a formula in stead of the entity name + GUID:

And the formula is:

If anyone has a better solution, please let me know :-).