How to pass String to constructor from a map

I'm trying to create an Athlet Object from a DocumentSnapshot. In this DocumentSnapshot I have stored a map, that holds the values(i.E. "upperbody": 0), to this athletes favourite Categorys. But for the AthleteObject I only need the key, of the category with the highest Value. This should work, but I get the following error. And I dont' know how to rewrite this for it to not complain.

The instance member 'getFavCategory' can't be accessed in an initializer. (Documentation) Try replacing the reference to the instance member with a different expression

Athlete.fromJSON(DocumentSnapshot parsedJSON):
        _name = parsedJSON["name"],
        _rank = parsedJSON['rank'],
        _xp = parsedJSON['xp'],
        _foot = parsedJSON['foot'],
        _position = parsedJSON['position'],
        

  String getFavCategory(Map<String, dynamic> map){
    String favCat;
    int maxVal;
    map.forEach((key, value) {
      if(maxVal == null || value > maxVal){
        maxVal = value;
        favCat = key;
      }
    });
    return favCat;
  }

Your definition of getFavCategory states that it has to be called on an initialized instance of your Athlete class (which is not a case for your constructor).

The fastest you can do is simply turn your getFavCategory function in to static (since you are not using any class variables inside):

static String getFavCategory(Map<String, dynamic> map){
    String favCat;
    int maxVal;
    map.forEach((key, value) {
      if(maxVal == null || value > maxVal){
        maxVal = value;
        favCat = key;
      }
    });
    return favCat;
  }