Cannot start Minecraft 1.12.2 after playing on 17w43a

After playing on the new Minecraft snapshot (17w43a), the launcher stops me playing the game. Here is the game output for Vanilla 1.12.2:
OpenJDK 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
Error: Could not find or load main class net.minecraft.client.main.Main
Is there a patch for this? If so, please can you help!

Also starting any other version does the same.