This could happen if objects were hidden after going to editmode and
knife was used on them.
In that case, raycasting would (rightfully) fail, but there is a
fallback in place using the back-buffer selection method (in which the
hidden object is still present). So a face would be found, but this
makes all following code confusing/wrong since we are working with
coordinates / faces under the assumption there would be a valid/
corresponding object to it in `KnifeTool_OpData` > `objects` -- which is
not the case...
So to resolve, just check if the object is visible before calling
`EDBM_face_find_nearest` in knife code.
Alternativeliy, we could also add a check for Base viewport visibility
to all the bmesh `find_nearest` functions (a bit hesitant though since i
am not sure this would be desired in all cases)
Philipp Oeser