At least when using the sketch widget, upon drawing a circle, the object's geometry no longer has a radius property. This was not the case in 4.11 version of the API, where you did have access to the radius property after drawing a circle. Don't know if this is the case with all circle objects in the API....
Reproducible @ https://codepen.io/crackernutter/pen/LKrmQb
Change codepen API to 4.11 to see the changes....