BUG: fix null pointer crash in view to world with no active camera

When Slicer layouts change, the slice node can be set and trigger an
update of widget positions before the renderer is fully set up. This
change avoids a null pointer crash caused by the renderer not checking
before dereferencing the ActiveCamera pointer. Match the error message
printed by the vtkOpenGLRenderer, return 0,0,0.

Slicer Issue #3761