My MInecraft Launcher Keeps Crashing

I have a mac and everytime I go the minecraft launcher, I press play, and it starts to load but it won't actually open minecraft. Then the launcher says

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)

I don't know what to do! Please help!