So the point is, that Minecraft is instantly crashing in the new snapshot 17w50a
Btw: im using a mac os with java Installed, if that is helpful.

The game crashed whilst initializing game
Error: com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Unterminated object at line 4 column 28 path $.pack.description

If im clicking on the show crash report nothing happens...

But in the output log it says following:

18:42:29 vf Client thread error Something went wrong upgrading!
java.lang.StringIndexOutOfBoundsException: String index out of range: -1
at java.lang.String.substring(String.java:1931)
at xc.a(SourceFile:20)
at vf.b(SourceFile:55)
at vf.a(SourceFile:40)
at bnh.a(SourceFile:1004)
at bnh.a(SourceFile:776)
at bnh.<init>(SourceFile:373)
at bne.an(SourceFile:404)
at bne.a(SourceFile:365)
at net.minecraft.client.main.Main.main(SourceFile:140)