Dart - Map with List as a value - flutter

Question

I have this:
List<ForumCategories>? forumCategories;
var mainCats = List<ForumCategories>.empty(growable: true);
var subCats = <int, List<ForumCategories>>{};
Than this is what I do in order to populate it:
for (var i = 0; i < forumCategories!.length; i++) {
if (forumCategories![i].main == 0) {
subCats[forumCategories![i].slave]?.add(forumCategories![i]);
}
}
And than when I do:
var size = subCats.length;
print("SUBCATS: $size");
Output is always: SUBCATS: 0
This does not populate the map. Any idea why ?

Answer 1

because you're trying to add an item to a null List, see here:
if (forumCategories![i].main == 0) {
subCats[forumCategories![i].slave]?.add(forumCategories![i]);
}
subCats[forumCategories![i].slave] does not exist yet in your map, it's like you saying:
null.add(forumCategories![i]);
nothing will happen.
you need to initialize it by an empty List if it's null like this:
if (forumCategories![i].main == 0) {
(subCats[forumCategories![i].slave] ?? []).add(forumCategories![i]) // this will assign an empty list only the first time when it's null, then it adds elements to it.
}
now before adding elements to that list it will find the empty list to add.