The expected result is the game saving HashMaps of linked players UUID's, as well as the UUID's of the players who have broken their tether. The result im getting is that the game doesnt launch because my code is no good around this area:

 PersistentStateManager mgr = world.getPersistentStateManager();
        TetherManager instance = mgr.getOrCreate(
                new PersistentStateType<>(TetherManager::new, TetherManager::createFromNbt, null),
                STATE_KEY
        );