Did you build a migration microflow as described in this step?
3.1 Back Up Your App
If you need to preserve the data in your current running app, make a backup! This migration will make changes to system module entities and you will need to adjust your own entities as well, so we expect data to be lost.
3.2 Build Data Migration Microflow
Build data migration microflows if necessary
And is this nullpointer also when starting the workflow from the beginning or an already created earlier task?
What are your security setting for the workflow instance entity? Is the user which triggers workflow has roles compatible with the security settings on workflow instance entity?