It seems like getSource() will return null from the AreaEffectCloud created by lingering potion fired from dispenser. getSource() should return a ProjectileSource, which should lead to the dispenser block.

Test with this code:

List<Entity> nearby = player.getNearbyEntities(32, 32, 32); 
for (Entity entity : nearby) { 
    if (entity instanceof AreaEffectCloud) { 
        AreaEffectCloud area = (AreaEffectCloud) entity; 
        player.sendMessage(entity.getType().name() + "'s source: " + area.getSource()); 
    } 
}

A test plugin is attached. Use command "/taec" to show nearby AreaEffectCloud nearby. The screenshot represents two lingering potions being thrown: One by myself, and one by dispenser.

On the other hand: getShooter() from Arrows fired by dispenser works fine.