In the assignments tool, some process is able to create assignments bypassing the open/due/close dates being set. I'm not sure yet if this is via lessons, via import or via template (my best guess).
Someone today on nightly did it and a few of our clients have done it. This results in the UI error:
This is because there is no NPE checks in getAssignmentStatus for open/due (there is for close) and also in the method getAssignmentCannonicalStatus. Ideally these would be checked to avoid this error, but also whatever is inserting these incorrectly should probably also be discovered so nulls don't end up in the database.
I fixed this for the client by just setting all the open dates to now, and all of the close/due to a week out similar to the UI.