Categories None in activity metadata

[MaximeAgez]

Hi.
Whenever I use wurst.extract_brightway2_databases() the resulting list incorporates a key categories: None in the activity description. This creates an issue whenever at the end you try to write_database not using wurst.
I know that it's not the expected function to use (should use wurst.write_brightway2_database() but because of reasons I have to use the brightway2 function write_database().
Anyways, while using the write_database() of brightway2, it tries to join categories (for elementary flows) but then also tries to do the same for activities (since wurst added categories) and because it is None, the .join of the _format_dataset() function of the IndexManager of brightway2 fails.
In any case I already resolved my issue but i still flag it instead it's somehow useful.