For OOP iframes, a doc load complete event on the embedder document doesn't necessarily mean the iframe document has finished loading yet.
We already checked for the busy state on the nested iframe doc and waited for a doc load complete event in that case.
However, depending on timing, it might be possible there is no document at all (even a busy one), so we should wait for doc load complete in that case too.
This is a speculative fix; I wasn't able to reproduce this myself.
Differential Revision: https://phabricator.services.mozilla.com/D118123
165b6e2e26