OS: Mac OS X (ver 10.9, arch x86_64)
Java: 1.6.0_65 (by Apple Inc.)
Launcher: Minecraft Launcher 1.3.1 (bootstrap 4)
Minecraft: 1.7.2 (updated Fri Oct 25 15:00:00 CEST 2013)
1. Went to the nether, dug about 250 blocks away from the first portal.
2. Build the second portal and activated, went in the portal.
3. Moved around in the normal world for block rendering.
and then the game crashes
Put the summary of the bug you're having here
MC-30135 Hopper: java.lang.NullPointerException: Getting biome