canStart is not a function

Im trying to open a combo box on the same position of the button:
Bildschirmfoto 2022-08-20 um 18.31.20
So I overwrite the function of contextMenuTool, to set the position of object:

"toolManager.contextMenuTool": (contextmenu, obj) => {
                contextmenu.position = new go.Point(obj.position.x + 20.0, obj.position.y + 20.0);

Without Webix Library works, but when I activate Webix, than I receive following exceptions:

go.js:314 Uncaught TypeError: d.canStart is not a function
    at La.doMouseUp (go.js:314:199)
    at Q.doMouseUp (go.js:605:279)
    at a.Zj (go.js:822:147)

Do you know how I can fix it and why is this happen?

The value of ToolManager.contextMenuTool must be an instance of ContextMenuTool, not a function.

uff, yes you have right, I just should read the documentation ContextMenuTool | GoJS API now it works, thank you.