We are aware of a recent iOS update that has caused many iPhones to reboot when using apps that utilize local notification settings. Apple has released a fix for this issue, so you should see this resolved if you update to iOS 11.2 on your device.
You can read more about the issue here: https://9to5mac.com/2017/12/01/how-to-fix-iphone-randomly-rebooting/
Please see this help article for information on how to update the iOS version on your device: https://support.apple.com/en-us/HT204204
Thanks for your patience!