create any polygon (radius will be enabled and editable)
Create a square
Create another polygon and the radius is greyed out. This is also true for any subsequent polygons. Also the original polygon now has radius disabled as well.
If you create a circle then click on the polygon the radius will be available. But if you click back on the square again the radius will be grayed out again. click on the circle again and the radius will be available again. Has something to do with the square which has its radius grayed out.