how to add a null list to list of list <string>?

If history has no elements already, you can simply do:

history.Add(TP);

and the elements will automatically be given the correct index.

However, if history already has elements and you want to replace the one at index winc -1, you can do:

history[winc - 1] = TP;

Finally, if history already has elements but you want to insert one at index winc -1, you can do:

history.Insert(winc - 1, TP);

Notice that, if TP is not null, it should refer to a different List each time you add, replace or insert an element if you don't want elements to be duplicated.