error CS1061: Type `ListHashSet<BasePlayer>' does not contain a definition for `ForEach' and no extension method `ForEach' of type `ListHashSet<BasePlayer>' could be found. Are you missing an assembly reference?

private void DrawGUI(CuiElementContainer container) =>
BasePlayer.activePlayerList.ForEach(x => CuiHelper.AddUi(x, container));

private void DestroyGUI() => BasePlayer.activePlayerList.ForEach(x => CuiHelper.DestroyUi(x, Layer));

Simply use foreach not the method because activePlayerList used to be a List but now its a ListHashSet so doesnt contain a ForEach method anymore.