BUG: test for active camera before enabling fiducials

Removes the error message that world to view and view to world
can't compute result due to a missing active camera. Delaying
setting the handles and widgets to be enabled if there's
no active camera defined on their renderers seems to work well
since the next time through the update code once the layout
has stabilised and the renderers are updated, the seeds
will get positioned and enabled.

Issue 0003761

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23493 3bd1e089-480b-0410-8dfb-8563597acbee