Is your feature request related to a problem? Please describe.
When creating a new expense or editing an existing one clicking back doesn't require any additional confirmation by the user. This way changes might be lost even though the user wanted to save them.

Describe the solution you'd like
When changes were done on the edit expense screen show a confirmation dialog whether the user wants to save the changes or not.
The user can either discard or cancel the request to stay on the edit expense screen.