Fortunately you can ignore this error message.
At the end if the installation it checks to see IDM shoudl be restarted.
If IDM had been running then the check will work properly and IDM will be restarted.
If IDM was configured to be off then the test should return false and IDM would not be restarted.
The test, however, fails with a blank arguement if IDM had not been running. This results in the error that you are seeing. Fortunately this also the last thing that happens in the installation. SO you are fine, and can feel confident that the installation will work just fine for you. I will write up a DDTS issue and hopefully have this very minor issue fixed for S27.
Thank you for pointing out this problem to us.
marco