If you’re getting an error along the lines of:
Error: You cannot use the Render object without specifying where you want to render it' when calling method
You need to fix the declaration in the widget you’re creating, as you’ve missed something really simple.
Copy the constructor from a pre-existing widget to make sure you haven’t missed anything.
Also, make sure you’re specifying a valid ID within the DOM.
This just cost me an hour of my day, so I hope this saves you some time.