"Looks like you're using an unsupported modified version of the game."

Every time I use a non-vanilla version of the game (not including snapshots), like OptiFine, or Forge on 1.12.2, the game sends 2 messages.

1. Looks like you're using an unsupported modified version of the game.
2. The game crashed whilst rendering overlay.

Not sure why this is happening, but the game loads 3 quarters then says those messages. Why is this happening?