NullReferenceException

[Rysefoxx]

Hi I'm trying to create a simple gui based on the wiki. It looks like a panel is created. Unfortunately it is always fullscreen black. Also, I always get this error because the method that is called does something to a button that is kind of null.

[Error  :Il2CppInterop] Exception in IL2CPP-to-Managed trampoline, not passing it to il2cpp: System.NullReferenceException: Object reference not set to an instance of an object.
   at UniverseLib.UI.UIFactory.SetDefaultSelectableValues(Selectable selectable)
   at UniverseLib.UI.UIFactory.CreateButton(GameObject parent, String name, String text, ColorBlock colors)
   at UniverseLib.UI.UIFactory.CreateButton(GameObject parent, String name, String text, Nullable`1 normalColor)
   at UniverseLib.UI.Panels.PanelBase.ConstructUI()
   at UniverseLib.UI.Panels.PanelBase..ctor(UIBase owner)
   at ShopBot.MonoBehaviours.TestWindow..ctor(UIBase owner)
   at ShopBot.Gui.CraftingPanel.SubGui()
   at ShopBot.Gui.CraftingPanel.Create()
   at ShopBot.Gui.PanelManager.Toggle()
   at ShopBot.MonoBehaviours.GuiComponent.OnGUI()
   at Trampoline_VoidThisShopBot.MonoBehaviours.GuiComponentOnGUI(IntPtr , Il2CppMethodInfo* )

Window Class:

public class TestWindow : PanelBase
    {
        public TestWindow(UIBase owner) : base(owner) { }

        public override string Name => "My Panel";
        public override int MinWidth => 100;
        public override int MinHeight => 200;
        public override Vector2 DefaultAnchorMin => new Vector2(0.5f, 0.5f);
        public override Vector2 DefaultAnchorMax => new Vector2(0.5f, 0.5f);
        public override bool CanDragAndResize => true;

        protected override void ConstructPanelContent()
        {
            Text myText = UIFactory.CreateLabel(ContentRoot, "myText", "Hello world");
            UIFactory.SetLayoutElement(myText.gameObject, minWidth: 200, minHeight: 25);
        }
    }

Method that registers the UI 1x.

public override void SubGui()
        {
            UIBase myUIBase = UniversalUI.RegisterUI("TODO", Update);
            TestWindow myPanel = new TestWindow(myUIBase);
        }

public void Update()
        {
            //TODO
        }

I wonder. Where is the label text and secondly why this exception always comes.

Thank you very much for the help.

Result: