Material design error message on top of a pop up

We are using material design messaging guidelines in a complex system,

the problem: in case I have an issue with a form and the issue is a non critical one, I will show a banner,

In case I have a form in a pop up window, I will not close the window until the server returns an answer,

The question: what should I do in case there is a non critical error after submitting a pop up form?

The reason I am asking is because a banner on top of a pop up seems odd, it seems like it is not focused to the issue, but in fact it is outside the window of the issue.