See https://bryntum.com/products/gantt/docs/api/Core/widget/MenuItem#eventhandler-onItem
I'm not sure why the argument is of type 'object' when you clearly knows what data is passed, as it is described in the documentation. I believe 'checked' should be marked "optional".
We are refactoring our gantt implementation and trying to get rid of all our hacks to make this thing work. I now stumble upon all the typescripts bugs and shortcomings again.
