Hmm. I can’t really reproduce this error at all. Do you mean you get this error specifically when building, or in runtime (because OnValitade should not be called in runtime). I have tried with a variety of combinations (prefab, non-prefab, asset and/or bound graphs0, but all tests build without any issues.
Do you maybe have any other 3rd party (or your own) asset/code that somehow may affect serialization or assets loading?
Can you please provide a bit more info on your setup with these BTs? Are they prefabs, scene instances, bound/asset graphs? Do you somehow custom loading the graphs into the graph owners?