Null Pointer Exception Crash

Failed to start game

java.lang.NullPointerException
at javax.imageio.ImageIO.write(Unknown Source)
at bem.c(SourceFile:312)
at beo.b(SourceFile:134)
at bcd.c(SourceFile:429)
at net.minecraft.client.Minecraft.a(SourceFile:296)
at atr.a(SourceFile:56)
at net.minecraft.client.Minecraft.run(SourceFile:506)
at java.lang.Thread.run(Unknown Source)