There are occasions when Silent Phone crashes. Sometimes it crashes at launch, sometimes it crashes after a short period of use. Often this is resolved by simply clearing the cache and data for the app on an Android device.
Launch settings > apps > select Silent Phone and tap > tap Storage > clear cache and data.
Restart the device and test again. If this doesn't resolve the crashes then possibly a re-install of the app will be the solution.
Applies to Android only.