submit method
Validates all fields and calls NashForm.onSubmit if valid.
Implementation
Future<void> submit() async {
if (_formKey.currentState?.validate() != true) return;
_formKey.currentState?.save();
setState(() => _isLoading = true);
try {
await widget.onSubmit(Map.unmodifiable(_values));
} finally {
if (mounted) setState(() => _isLoading = false);
}
}