Comment 9 for bug 166254

If a gradient (or any other such object) we're referencing
is deleted, directly or indirectly, any URIReference objects
should pick that up and notify their owners that the current
referenced object is NULL. And then we should just render
that the same way we would if the referenced object had not
existed in the first place.

I don't yet understand why that isn't happening...

As far as the appropriate thing to do in terms of fixing up
such nested situations, probably the best thing is to move
or copy the gradient into defs.

I would be suspicious of sprinkling sanity checks
everywhere; that is likely a sign that we are not solving
the problem in the right place. The "right place" almost
always happens to be one or two places, not hundreds.