When using Options » Send Email for one invoice, the recipient is not automatically filled with the customer’s email address (see [1.5.9] Bug? Using “Send Email” on invoice does not pre-fill the recipient). However, when I click on Cancel then, the browser will tell me to fill out the “To Email” field. For some reason the cancel button is triggering the HTML5 form validation. It shouldn’t though 
As stated in the other thread the e-mail address field is filled correctly in my IP. However when i clear this field and press cancel I see the form validation jumping in. Never saw this before because all my customers do have an e-mail address. I think it should not be impossible to open the send e-mail form when the customer has no e-mail address configured. And cancel should always cancel whatever you were trying to do.