This is the 'safe and simple' aspect of the fix: prevent `node_warnings`
RNA property of the Node modifier to be overridable.
Its access is 100% not thread safe currently - and it makes no sense to
have this reuntime data overridable anyway!
Another side of the issue will be fixed in a separate commit, for main
only, as it affects quite deeply the behavior of viewlayer resync, and
fixes some unrelated logical issues in the current code.
89efaee069