Currently there’s a bug with iOS Messages where a single message is able to crash the iPhone, iPad and even the Apple Watch. This is due to its difficulty to render specific unicodes which results a crash or a reboot.
Apple has acknowledged the issue and they are working on a new software update. In the time being if you’ve received such message and couldn’t open Message properly, Apple wants you to ask Siri for help.
To be able to re-open Messages, Apple recommends the following steps:
1. Ask Siri to “read unread messages.”
2. Use Siri to reply to the malicious message. After you reply, you’ll be able to open Messages again.
3. In Messages, swipe left to delete the entire thread. Or tap and hold the malicious message, tap More, and delete the message from the thread.
So far there’s no timeline on the new iOS software update but if you want to prevent further random crashes, another temporary remedy is to disable lock scren notifications for messages.