Hopper: java.lang.NullPointerException: Exception in server tick loop

I have the same issue. This exception occurs several times a day. Yesterday, it happened 7 times and the server was down for several hours. I implemented an auto-restart shell script to minimize the impact.

On one occasion, the exception occurred with no one logged for the entire run session. Here's the log:

[12:22:19] [Server thread/INFO]: Starting minecraft server version 13w41b
[12:22:19] [Server thread/INFO]: Loading properties
[12:22:19] [Server thread/INFO]: Default game type: SURVIVAL
[12:22:19] [Server thread/INFO]: Generating keypair
[12:22:20] [Server thread/INFO]: Starting Minecraft server on 192.168.0.3:25565
[12:22:21] [Server thread/INFO]: Preparing level "swim2"
[12:22:21] [Server thread/INFO]: Preparing start region for level 0
[12:22:24] [Server thread/INFO]: Preparing spawn area: 0%
[12:22:25] [Server thread/INFO]: Done (4.690s)! For help, type "help" or "?"
[12:22:25] [Server thread/INFO]: Starting GS4 status listener
[12:22:25] [Server thread/INFO]: Starting remote control listener
[12:22:25] [Query Listener #1/INFO]: Query running on 192.168.0.3:25565
[12:22:25] [RCON Listener #2/INFO]: RCON running on 192.168.0.3:25575
[12:22:32] [Server thread/ERROR]: Encountered an unexpected exception
java.lang.NullPointerException
        at ml.c(SourceFile:130) ~[minecraft_server.jar:?]
        at net.minecraft.server.MinecraftServer.u(SourceFile:529) ~[minecraft_server.jar:?]
        at le.u(SourceFile:259) ~[minecraft_server.jar:?]
        at net.minecraft.server.MinecraftServer.t(SourceFile:441) ~[minecraft_server.jar:?]
        at net.minecraft.server.MinecraftServer.run(SourceFile:358) [minecraft_server.jar:?]
        at kt.run(SourceFile:617) [minecraft_server.jar:?]
[12:22:32] [Server thread/ERROR]: This crash report has been saved to: /Users/steve/Minecraft/Swim2/./crash-reports/crash-2013-10-12_12.22.32-server.txt

The related crash log:

---- Minecraft Crash Report ----
// On the bright side, I bought you a teddy bear!

Time: 10/12/13 12:22 PM
Description: Exception in server tick loop

java.lang.NullPointerException: Exception in server tick loop
        at ml.c(SourceFile:130)
        at net.minecraft.server.MinecraftServer.u(SourceFile:529)
        at le.u(SourceFile:259)
        at net.minecraft.server.MinecraftServer.t(SourceFile:441)
        at net.minecraft.server.MinecraftServer.run(SourceFile:358)
        at kt.run(SourceFile:617)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
        Minecraft Version: 13w41b
        Operating System: Mac OS X (x86_64) version 10.8.4
        Java Version: 1.7.0_07, Oracle Corporation
        Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
        Memory: 2387973664 bytes (2277 MB) / 3436183552 bytes (3277 MB) up to 4294967296 bytes (4096 MB)
        JVM Flags: 17 total; -Xmx4G -Xms3G -Xloggc:server_gc.log -XX:+UseG1GC -XX:+DisableExplicitGC -XX:MaxGCPauseMillis=1000 -XX:+UseFastAccessorMethods -XX:+UseStringCache -XX:+OptimizeStringConcat -XX:+UseCompressedOops -XX:MaxInlineSize=270 -XX:FreqInlineSize=5000 -XX:+UseBiasedLocking -XX:+UseNUMA -Xnoclassgc -XX:OnError=/Users/steve/Minecraft/Swim2/scripts/jvm_on_error.sh -XX:OnOutOfMemoryError=/Users/steve/Minecraft/Swim2/scripts/jvm_on_out_of_memory.sh
        AABB Pool Size: 3957 (221592 bytes; 0 MB) allocated, 3811 (213416 bytes; 0 MB) used
        IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
        Profiler Position: N/A (disabled)
        Vec3 Pool Size: 4326 (242256 bytes; 0 MB) allocated, 4190 (234640 bytes; 0 MB) used
        Player Count: 0 / 20; []
        Is Modded: Unknown (can't tell)
        Type: Dedicated Server (map_server.txt)

Still an issue in 14w07a.