How do I save a list for each player in the config?

I am writing a plugin that must store a list of players in the config, and under each player name, there is a list of players that that player has added as a friend. What methods would I use to see if a player has added a certain other player, and what method would I use to add a single entry to the list? I tried creating a list, setting the list to getConfig().getList("playername") and then doing list.add(args[0]), and then setting the config section to the list, but it just threw a nullPointerException and erased most of my config. Is there a method to create or directly add to a list inside the config? Please just ask if you need to see more code or get more information.
what I want config to look like:

Code:

Player1
- Player2
- Player3
Player2
- Player1
- Player3

 

@Nukreeper64 Post your code. My guess is that 'list' is null. Alternatively args[0] may not exist resulting in an ArrayIndexOutOfBoundsException. Post all relevant classes in their entirety and any stack-traces.

 

try setting the list to getStringList()

 

You can't modify the return value of getList(), as it returns a List<?>. You need to have it either cast to a type or use one of the built in functions such as @Vextricity suggested.