Flutter error type 'List<Object?>' is not a subtype of type 'List<String>'

I have this list:

List<dynamic> activityDays = [];

And this code:

getItems(AsyncSnapshot<QuerySnapshot> snapshot) {
  if (widget.user.uid.isNotEmpty) {
  // ignore: missing_return
  snapshot.data.documents.map<Column>((f) {
  if (f.documentID == widget.currentList.keys.elementAt(widget.i)) {
    f.data.forEach((a, b) {
      if (a == "activities") {
        List<dynamic> markMap = f.data['activities'];
        for (var element in markMap) {
          activityDays.add(element['days']);
        }
      }
    });
  }
}).toList();

And printing the list gives the result:
[[M, T, L], [T, T, L]]

But when I am trying to get an item from the list it gives me error:
type ‘List<Object?>’ is not a subtype of type ‘List’

I am using it like this:

GroupedCheckbox(
  itemList: activityDays.elementAt(i),
  checkedItemList: checkedItemList,
  onChanged: (itemList) {
    setState(() {
      selectedItemList = itemList;
      print('SELECTED ITEM LIST $itemList');
    });
  },
  orientation: CheckboxOrientation.VERTICAL,
  checkColor: Colors.blue,
  activeColor: Colors.red,
),

What am I doing wrong?