I need to add a custom class to a forms field if validation fails.

This post is exactly what I need but it does not work - https://our.umbraco.com/forum/umbraco-forms/90433-umbracoforms-themes-set-error-class-on-wrapper

CMS: 8.14.4 Forms: 8.7.6

@functions {
public bool HasErrors(string fieldId) {
    if (ViewData.ModelState.ContainsKey(fieldId)) {
        return ViewData.ModelState[fieldId].Errors.Any();
    } else {
        return false;
    }
}

public string ValidStateClass(string fieldId) {
    var klass = "";
    if (HasErrors(fieldId)) {
        klass = "invalid";
    }
    return klass;
}

}

@ValidStateClass(f.Id)

Does anyone know how to fix or have an alternative solution?

Thanks