*<Null safety>*
T? singleWhereIndexedOrNull (bool test(int index, T element))
T? singleWhereIndexedOrNull(bool Function(int index, T element) test) { T? result; var found = false; var index = 0; for (var element in this) { if (test(index++, element)) { if (!found) { result = element; found = true; } else { return null; } } } return result; }