The error appears simply when selecting a cell in the first grid of the demo.
It also appears when editing a cell in other grids.
The error message on my machine is:
TypeError: string indices must be integers ... File "xxx\lib\site-packages\st_aggrid\__init__.py", line 171, in AgGrid frame = pd.DataFrame(component_value["rowData"])
On https://share.streamlit.io the message is:
Whoops — something went wrong! An error has been logged.