Prevent submission in no targets selected

[1v4n4]

#1478

[tttp]

I'm not a fan of your approach, could you either

• add an hidden field "targetCounts" with a custom validation that set the value to the number of targets and returns an error if missing

• or use the resolver:
  and use props.extraValidation (as we do for extraFields)

  resolver: async (data) => {
  // Perform normal validation using the default resolver
  const { errors } = await validate(data);
  const extraErrors = props.extraValidation && props.extraValidation(data);
  if (extraErrors)
  return {...errors, ...extraErrors};
  // Return the errors
  return errors;
  }