Verify last instance of a textfield is not null

I have a repeating subform that includes a textfield for the user to enter their name. When the user clicks a custom email button to send the form I want to first verify if the last instance of that field was populated (not null). I don't care about the previous instances only the last instance. The custom email button is pushed down below each instance of the subform so it is always just below the last instance of the repeating subform. Is there a way to refer to the last instance of a subform field when i don't know how many instances will be created?

Thanks