You got any crash report or log so we can see the source of the error? Do you force-update when you reinstalled? (deleted .minecraft and tried to play minecraft via new minecraft launcher, so it downloads everything)
Are you running a mod? If yes, what mods? Try to give us maximum info so we can find the source.
1. I'm not sure if this is useful, but here is the output log:
16:31:32 launcher Preparing to launch minecraft client for 1.12.2 16:31:34 launcher Checking installations. 16:31:34 launcher Minecraft client 1.12.2 is ready to start. 16:31:34 launcher Starting! 16:31:34 launcher Using default game log configuration client-1.12.xml (outputs XML)
2. Yes, I did delete everything in .minecraft.