I can't get back to 1.12 or 1.12.2 after opening 17w43a

After opening Minecraft in 17w43a and quitting, I can't reopen Minecraft in 1.12 because:

Erreur : impossible de trouver ou charger la classe principale net.minecraft.client.main.Main
Java HotSpot(TM) Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release