I have a modifiable list (I did not set the list to have a fixed size) but run into this error

Unsupported operation: Cannot add to an unmodifiable list

The source code that throws this error is:

for (int i = 0; i < globals.produkte.length; i++){
   globals.produkte[i].kategorienBool.add(false);
}

kategorienbool is a List of bool with no fixed size.

This is how kategorienBool is defined in my class Produkte:

List<bool> kategorienBool;

Produkt({
    required this.zutat,
    required this.menge,
    this.mhd,
    this.verfaelltInXTagen,
    this.einheit = "g",
    this.anlage,
    this.kategorien = const ["keine"],
    this.kategorienBool = const [false],
    this.offen = false,
    this.color = const Color.fromRGBO(0, 0, 0, 1)
  });

Can someone give me a hint what I might be doing wrong?

I also red that this seems to be a problem other people already had within the last 4 weeks AND I even found github posts from people which had this issue like 2 years ago but it had been fixed at that time.

The question is now: Is this a new issue or am I just doing something wrong?