ok - i am on a mac - now your script works.
there is a sample for a c# plug-in command with history:
check this topic
and
As soon as you add an Object to the document a user can edit your circle …
what s the exact usage-Scenario you re after ?
Maybe a Display Conduit will fit better for your needs - this will allow you to draw geometry to the screen to preview the result of your dialog - and as soon as the command / dialog is finished, add the final object to the document ?