Minecraft 1.7 Beta error

Extending piston into a stream of water triggers the below error and Saving chunk appears then minecraft closes and the world refuses to open afterward.

java.lang.NullPointerException
        at ut.a(SourceFile:130)
        at uk.a(SourceFile:97)
        at uk.n_(SourceFile:145)
        at fd.g(SourceFile:1119)
        at net.minecraft.client.Minecraft.k(SourceFile:1494)
        at net.minecraft.client.Minecraft.run(SourceFile:754)
        at java.lang.Thread.run(Unknown Source)
Stopping!