Blender crashes in older versions when loading files saved from v4.5 and
contain nodes like Mix in the compositor. That's because those nodes do
not exist in older versions, that is, this is a forward compatibility
issue.
The compositor already has protections in place to not crash when
loading files from future versions containing new nodes. However, for
nodes like Mix which existed in other node trees, the node is not new,
it is just new to the compositor, so it goes undetected and the
compositor tries to execute it leading to a crash.
To fix this, we also check the poll method of the node when verifying
compositor node trees. This should be backported to the corrective
release, as well as LTSs. But for v4.5, a change will later follow to
make the compositor execute such undefined/unsupported nodes using a
default operation that initializes the output with default values.
Pull Request: https://projects.blender.org/blender/blender/pulls/136438
577c46c081