When you use a bug in Message app, it will attempt to load a large amount of data that couses the system malfunction. However the text/file is very complicated for the system and causes a CPU average and freezing the app. If you close the app and try to reopen it, iOS will try to reload the previous message but can’t because it’s the bug and the system can't handle it properly.
⚠️ Kill the app in appswitcher before this ⚠️
If your Message app crash, click here
Alternate fix 1
Alternate fix 2