If you deactivate Javascript in Safari Settings > Security or in the Developer menu. When you open Webinspector after JS deactivation for the first time it doesn't work. Open Webinspector before deactivate JS execution and it works as well. (Think this is an other bug in JS code execution). I think this is a big problem because the Webinspector is JS based an for Safari it is only a further page. So i don't know if possible to change Safari to a more page based Javascript deactivation. But i think it would be nice if it is possible, in future versions, to use Webinspector also with deactivated JS.
*** This bug has been marked as a duplicate of 12693 ***