'Tools → Export as STEP' fails

I used the [Assembly Tutorial](https://cadquery.readthedocs.io/en/latest/assy.html), which I got to load after downloading 'vslot-2020_1.dxf' following a link from a related [GitHub Issue](https://github.com/CadQuery/cadquery/issues/589).

Version: 0.3.0dev.

'Log viewer' reports:
```
[2021-04-14 16:55:01.679632] ERROR: CQ-Editor: Uncaught exception occurred
Traceback (most recent call last):
File "C:\Users\user\Anaconda3\lib\site-packages\cq_editor\widgets\object_tree.py", line 142, in
self.export('step'))
File "C:\Users\user\Anaconda3\lib\site-packages\cq_editor\widgets\object_tree.py", line 332, in export
export(shapes,export_type,fname,precision)
File "C:\Users\user\Anaconda3\lib\site-packages\cq_editor\cq_utils.py", line 76, in export
comp = to_compound(obj)
File "C:\Users\user\Anaconda3\lib\site-packages\cq_editor\cq_utils.py", line 37, in to_compound
raise ValueError(f'Invalid type {type(obj)}')
ValueError: Invalid type
```