Game crashes when opening a second instance of a world during the loading process of the same world

For corrupted worlds, please see MC-2545

Crash: During the loading process of a world, Minecraft switches briefly to the main menu (see MC-185).
If you start loading a world, that takes a long time to load (due to corrupted chunks, see attached world below), when Minecraft switches to the main menu it is now possible to open a second instance of the same/another world, that will lead to the crash attached below.

Steps to reproduce:
1. Download the attached world below.
2. Start it.
3. When the game switches to the main menu quickly open the same or another world.
Minecraft crashes with the crash attached below.

Original Description:

• After updating to 1.4.5, I logged in and presumed play.
• At one point, an invisible (and unbreakable) block blocked my path. It was after I navigated a path around it that I noticed some serious lag.
• I restarted my computer, hoping this would solve things.
• Selected my save file and proceeded to load into the game
• The game returned me to the main menu screen
• I reselected my save file and waited during the loading screen
• This is where the game crashes! Please see attached txt.

*Note

• If I select my save file (the first time), and wait about 20 seconds (after being kicked back to the main menu) the game actually loads! However, it is impossible to play because of intense lag.
• I have no mods installed, and I was on single player!

Basic cause: Corrupted chunks, see MC-2545.

I hope this is enough information!