can't add "Panel"

[cipsicko]

Hi,

this is a piece of my code.

//== PRELOAD STATE
...
preload() {
        //== SlickUi
        slickUI = Game.plugins.add(Phaser.Plugin.SlickUI);
        slickUI.load(kenney.json');
    }
...

//**
   * MENU STATE
   *  the "Preload State" call the "Menu State" where i create the "Panel" element.  
   * "_G.SLICKUI" is a global variable
*/
...
create() {
        let panel;
        console.log(SlickUI.Element); // ==> 'Undefined'
        _G.SLICKUI.add(panel = SlickUI.Element.Panel(8, 8, 150, 16)); // ==> Cannot read property 'Panel' of undefined
}
...

I can't figure out how to solve it.
If call SlickUI.Element.Panel in the console it returns a function (so it should work), but return Undefined if called inside create() method