The little helper to-do widget shows "missing N categories for <present month>'s budget" when viewing past months that are missing values for categories.

To reproduce this, set a budget for all categories in the present month (as of writing, July), then go back to June, which is missing values for one category. You should see the message as indicating July is missing categories.

Solutions could be to just skip this message for past months, or update the error to indicate the viewed month instead of the present month.