Fix form validation in event registration

Closes #87

Previous behaviour

App crashes when an integer field is emptied.

Steps to reproduce:

1. Open an event registration with a required integer field.
2. Delete the value.
3. Notice the app crashes.

New behaviour

App no longer crashes when an integer field is emptied. Also, it now correctly handles required boolean fields.

Steps to validate that it works:

1. Open an event registration with a required integer field.
2. Delete the value.
3. Notice the app no longer crashes, and instead adds a notice that the integer field is invalid.