reset method
Resets every FormField that is a descendant of this Form back to its
FormField.initialState
.
The Form.onChanged callback will be called.
If the form's Form.autovalidate property is true, the fields will all be revalidated after being reset.
Implementation
void reset() {
for (FormFieldState<dynamic> field in _fields)
field.reset();
_fieldDidChange();
}