There's a new bug going around that can crash iPhon'es messages app. Cult of Mac is calling it the "black dot of death" but it's not actually the black dot emoji the causes the damage. It's a thousands-character-long string of invisible Unicode text that accompanies the dot that causes the problem. The bug affects iOS 11.3 and 11.4 betas.
The only fix is to use 3D Touch on the messages icon, according to EveryThingApplePro on Youtube.
So, don't use the black dot emoji!