isListOfList function
Returns true
if list
elements are all of type List.
Implementation
bool isListOfList(Object? list) {
if (list == null || list is! List) return false;
if (list is List<List>) return true;
if (list.isEmpty) return false;
if (listNotMatchesAll(list, (e) => (e is List))) return false;
return true;
}