The crash in TransformAccessClearCallback comes from TransformAccess data that was saved into Undo being changed without updating the Undo System. This happens during a call to a user function called RebuildLock called after the undo data has been finalised. When attempting to undo, none of the relevant Transforms exist any more and clearing/applying the serialised data regarding those Transforms fails.

Any code where new objects are created but not explicitly saved into the Undo System cannot be considered fully undoable. Additionally the Undo System cannot be expected to recover gracefully when the data it was provided is incomplete or incorrect, it is simply pointing to invalid memory once something changes without it being informed.

In this case the constructor OnewayArmatureLock creates hundreds of unsaved Transforms and then baseBonesAccessor is switched to this list, making the TransformAccessArray data that was saved to the Undo System invalid.

The crash described can be avoided by registering all the 'baseBone' Transforms using Undo.RegisterCreatedObjectUndo(baseBone, "") to ensure that the Transforms referred to in the TransformAccessArray contained in the Undo action still exist.

Alternatively if the _baseBonesAccessor TransformAccessArray is disposed using _baseBonesAccessor.Dispose() before being directly overwritten the memory is cleared correctly and the Undo system is updated accordingly.