User is trying to email an invoice to the client from the preview window, but the email address of the client does not auto-populate in the To field of the Email dialog.
This is the expected behavior in Core. The email address does not auto-populate from the preview window. However, if you email the invoice from the list view using Actions > Email, the email address of the recipient is auto-populated in that case. Check Core Help Center for details on emailing invoices.