Assigning a list to a list of lists

public List<List> squads = null;
private int numsq = 0;
public List _selectedUnits;

Problem code:
if(Input.GetKeyDown(KeyCode.F) && _selectedUnits != null){
squads[numsq] = _selectedUnits;

I’m trying to implement squads into my RTS. I used the multiple selection code here:

And that works fine, but when I hit F it comes back with an error pointing to the list assignment above:
NullReferenceException: Object reference not set to an instance of an object

Help please!

Looks to me like squads is null. You need to assign it somewhere before that line with something like; squads = new List().