Crashes when you hit play on launcher.

Whenever I hit "play" on the launcher, it closes down as normal, but when the normal Minecraft icon pops up, it just crashes and I get this for the feed:
[09:45:33] [Client thread/INFO]: Setting user: TheKiwiGamerYT
[09:45:33] [Client thread/INFO]: (Session ID is token:c1a0a8fc685f4d10bf16957ae06e3cef:dfedddca0b9043b18cc9b781c09d381b)
Exception in thread "Client thread" java.lang.NullPointerException
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:213)
at com.google.common.collect.Lists$ReverseList.<init>(Lists.java:767)
at com.google.common.collect.Lists.reverse(Lists.java:759)
at lt.b(SourceFile:156)
at lt.<init>(SourceFile:43)
at net.minecraft.server.MinecraftServer.<init>(SourceFile:155)
at bpo.<init>(SourceFile:41)
at ave.<init>(SourceFile:293)
at net.minecraft.client.main.Main.main(SourceFile:124)