I brought up a URL-based PDF in a DEVON browser window, clicked the center of the window to select it, and ran the script. The AppleScript even log says:
tell application “DEVONthink Pro”
get URL of window 1
“http://www.umass.edu/philosophy/events/papers/Stang-Job%20Talk.pdf”
get selection of window 1
“DEVONthink Pro got an error: Can’t get selection of window 1.”
Great job! 
…and great feedback 
Just run the script in a main window using split/three-pane views, not in its own window. Or use this script: http://www.devon-technologies.com/scripts/userforum/viewtopic.php?f=20&t=5882&p=26652
Thanks! The “not in its own window” restriction was totally unexpected. I wonder why it exists? 
I guess this statement answers the question 
“Please note that all these scripts have not been developed by DEVONtechnologies and come without any warranty. Download and use at your own risk!”
Anyway, you won’t have to use a script anymore as soon as v2 will be available.