I think it is because you've made the UI script an 'autoload' script.
Autoload scripts are loaded before anything else in the scene tree has loaded. So, when you write
onready var LL = ...
it means that the engine will try to assign the Label node to the LL variable as soon as UI has been loaded (which is before any other nodes). Result : the engine cannot find the Label node, so it assigns 'null' to the variable.
If you really want the LL variable to exist inside the UI script, you could let the Label node register itself. The UI script has :
var LL ( ! not 'onready')
And the Label script class has :
UI.LL = self
The _onready() function fires as soon as the Label node enters the scene tree.
UI, which is an autoload script, is available from anywhere in your project ( except from other autoload scripts which are loaded before it ).