Dev Launcher won't start the game

When logging in using the new MC Launcher on OSX, the game starts with the MOJANG splash and then crashes.

Output:-

Client> java.lang.StringIndexOutOfBoundsException: String index out of range: -1
Client> at java.lang.String.substring(String.java:1937)
Client> at asm.a(SourceFile:1469)
Client> at asm.a(SourceFile:1461)
Client> at asm.k(SourceFile:1452)
Client> at asm.K(SourceFile:300)
Client> at asm.c(SourceFile:503)
Client> at net.minecraft.client.main.Main.main(SourceFile:76)
Client> b@2f7574b9