Crash trying to add filter after deleting one

[mvglasow]

Steps to reproduce:

1. Touch “Create Rule”, and select an event.
2. Click “Add Filter” and add a filter (e.g. time of day).
3. Long-press the filter, choose “Delete” and confirm.
4. Click “Add Filter” again.

This will cause a crash of the app.

Tested with 0.5.0 (from F-Droid) on Android 4.4.4 (CyanogenMod 11-20141112-SNAPSHOT-M12-crespo).

The same issue occurs on a private branch based off 43c11b4 running on Android 7.1.1 (LineageOS 14.1.20170123-NIGHTLY-bacon).