New Account wizard; when First Payment Period set to "On Date" the date might show as blank

In the Account Quotation form and New Account wizard, when the First Payment Period set to "On Date", the first payment date might show as blank.

To recreate:

1. Enter a new Account.
2. Set the First Payment Period to "On Date".
3. Enter the First Date as the current date.
4. Change the Opening date onwards a few days.

At this point the First Date appears blank. Clicking up/down in First Date does nothing. This is because the First Date is prior to the Opening Date and therefore is invalid.

This behaviour has been changed so the First Date retains an invalid date, but when clicking "Calculate" a message to this effect is shown.