Update RigidBody to make sure Shapes collection can never be null
or empty.
Bugfixes
#FIX: Fixed rare serialization bug that sometimes caused reference Types with custom equality comparisons to lose their identity and be accidentally merged together.
#FIX: Fixed Scene information in the editor sometimes not being updated correctly after saving a previously unsaved Scene.
#FIX: Fixed a bug when renaming content refs.

If some content had been added to the editor, but the type for that
content was missing (not checked in yet/plugin folder not updated
yet etc), the editor would throw a confusing exception. Added a helpful
warning message and a null check.
Bugfixes
#FIX: Fixed Dictionary cloning behavior, which failed to provide cloned key-value pairs for reference types.