Customer in constraint

I would like to know if it is possible to verify the presence of a customer , before printing an invoice ,
I can use the customer in constraint?

@gioalba it is really important for us to understand your use case. There might be 2 or 3 different answers for your question depending on how you’ll want to use it.

Quality questions can receive quality answers. Here is an example question. It is really very easy to understand what he is asking for.

thanks @emre. When I print an invoice i must have selected a customer .
I would sambapos stop print if i did not select any clients .
I 'm trying so

but I think that is not correct the constraint in the rules

Seems fine but casing for phone custom field might be incorrect.

{ENTITY NAME:Customer} Is Not Null, It seems to work

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