If your app is crashing or freezing, restarting your device can help resolve this issue. If it persists, there are a few additional steps you can try:
1. Update your iOS to its most recent version (keep in mind that beta versions aren't supported).
2. If you experience freezing/crashing issues with other apps, it may be an issue with your mobile device. In this case, your local Apple Store can help.
1. Update your Android OS. It is possible that the issue is related to an older version the operating system and this can help resolve the issue.
2. Force stop the app. Some apps may be still using some of the background services after you have closed them, and the Force stop feature can completely shut them down.
You can find more information on Android troubleshooting here.