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.