submit method
void
submit()
Implementation
void submit() {
var isValidForm = _formKey.currentState!.validate();
if (!isValidForm) return;
FocusScope.of(context).unfocus();
Map<String, String> data = {};
for (var f in widget.fields.values) {
if (f is AutoGroupField) continue;
data[f.id] = f.value;
}
widget.onSubmit(data);
}