Don’t worry about that unfortunate string of characters that can crash your iPhone with only a simple text message, Apple acknowledges the error, and is already working on a fix. Meanwhile, for those who are affected by the bug, here’s a temporary solution shared by Apple.
For those whose iPhone Messages app has crashed after you received that text message, all you need to do is tell Siri to “read unread messages”. Once Siri is done, she will ask if you would like to reply to the message, simply reply to the malicious message, and you will be able to load Messages again. Once done, open Messages, swipe left to delete the entire thread, or tap and hold the malicious message, tap More and delete the message. This help is posted into Apple’s Support site.
Other methods suggested by many users on the internet includes telling Siri to send that sender a message, opening a photo and share it to the sender, or if you received an iMessage, simply load the Messages app on your Mac to reply to that message.
Apple says that they are patching up the bug and the fix will be available via a software update. Until then, since there’s nothing you can do about it, just remember these steps.