Loading and reloading STEP would be crash

Example
https://drive.google.com/file/d/1uNoSakAb8fRtFri1-0YIOQsNMPdAX5u1/view?usp=sharing

Just open `Untitled.ipynb`. There are many empty item in my original model.

> In `items_occ` : LIST[( , str(model name), tuple( int(R), int(G), int(B) ), str(model tree path) )]
> In `items_cq_ocp` : LIST[( , str(model tree path) )]

But I have the problem.
In this line :
```python
assembly.save('./assembly_by_cq.step')
```
I tried to open `assembly_by_cq.step` by CREO 6.0. It has crash like this :

It would have same problem when I save STEP first and then reload STEP in cadquery.
The origin:
The reload: