When loading an unknown node type from a newer Blender version, the node
storage data cannot be properly loaded. Re-saving the file will then
crash if saving with the same node type idname, since new Blender
versions cannot find the expected storage data.
Loading in older versions should replace unknown node types with a dummy
"Undefined" node that will get loaded as `NodeTypeUndefined` in newer
versions as well. Custom node types are exempted from this since they
store all data as generic ID properties and can always be fully
serialized.
Lukas Tönne