Removing categories

[simphax]

Removing categories hides all budgetitems and transactions from the lists and opening graphs makes the app crash.

Steps to reproduce:

1. Add a category
2. Add a budget item or transaction item with the just created category
3. Remove category

Result:
All transactions and budget item with the category disappears. Opening the graph view crashes the app

Expected result:
All transactions and budget items with the category should change to another category.