User cannot discard invalid changes

Invalid changes to component values cannot be discarded.

Steps to reproduce:

1. Edit the Christmas Tree component on the 'cprops' page of the HAP
2. Make the number field invalid (e.g. enter some letters)

Expected: the 'Discard' button becomes enabled once there are changes. Clicking it discards the (invalid) changes so far and sets the form back to the initial state.

Actual: the 'Discard' button remains disabled, so the invalid changes cannot be discarded.