When baking custom properties, avoid keeping references to custom
property values that are known to be impossible to animate anyway.
The crash was caused by custom properties containing collections of ID
properties. Keeping Python references around for too long and then
accessing them caused Blender to crash.
My solution is to only keep track of custom property values that might
be keyable. For some this is certain: floats, ints, bools are keyable,
whereas lists, dicts, etc. are not. However, strings can be the RNA
value for an enum property, which is keyed via its integer
representation. So, the new function `can_be_keyed()` can return `True`,
`False`, or `None`. By skipping those values where it returns `False`
the crash is already resolved, making it good enough for now.
Pull Request: https://projects.blender.org/blender/blender/pulls/117993
25bdaf026c