TestAssignmentManager sometimes fails

Stacktrace

java.lang.Exception: test timed out after 5000 milliseconds
	at java.lang.System.arraycopy(Native Method)
	at java.lang.ThreadGroup.remove(ThreadGroup.java:969)
	at java.lang.ThreadGroup.threadTerminated(ThreadGroup.java:942)
	at java.lang.Thread.exit(Thread.java:732)
...
2012-10-06 00:46:12,521 DEBUG [MASTER_CLOSE_REGION-mockedAMExecutor-0] zookeeper.ZKUtil(1141): mockedServer-0x13a33892de7000e Retrieved 81 byte(s) of data from znode /hbase/unassigned/dc01abf9cd7fd0ea256af4df02811640 and set watcher; region=t,,1349484359011.dc01abf9cd7fd0ea256af4df02811640., state=M_ZK_REGION_OFFLINE, servername=master,1,1, createTime=1349484372509, payload.length=0
2012-10-06 00:46:12,522 ERROR [MASTER_CLOSE_REGION-mockedAMExecutor-0] executor.EventHandler(205): Caught throwable while processing event RS_ZK_REGION_CLOSED
java.lang.NullPointerException
	at org.apache.hadoop.hbase.master.TestAssignmentManager$MockedLoadBalancer.randomAssignment(TestAssignmentManager.java:773)
	at org.apache.hadoop.hbase.master.AssignmentManager.getRegionPlan(AssignmentManager.java:1709)
	at org.apache.hadoop.hbase.master.AssignmentManager.getRegionPlan(AssignmentManager.java:1666)
	at org.apache.hadoop.hbase.master.AssignmentManager.assign(AssignmentManager.java:1435)
	at org.apache.hadoop.hbase.master.AssignmentManager.assign(AssignmentManager.java:1155)
	at org.apache.hadoop.hbase.master.TestAssignmentManager$AssignmentManagerWithExtrasForTesting.assign(TestAssignmentManager.java:1035)
	at org.apache.hadoop.hbase.master.AssignmentManager.assign(AssignmentManager.java:1130)
	at org.apache.hadoop.hbase.master.AssignmentManager.assign(AssignmentManager.java:1125)
	at org.apache.hadoop.hbase.master.handler.ClosedRegionHandler.process(ClosedRegionHandler.java:106)
	at org.apache.hadoop.hbase.executor.EventHandler.run(EventHandler.java:202)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
	at java.lang.Thread.run(Thread.java:722)
2012-10-06 00:46:12,522 DEBUG [pool-1-thread-1-EventThread] master.AssignmentManager(670): Handling transition=M_ZK_REGION_OFFLINE, server=master,1,1, region=dc01abf9cd7fd0ea256af4df02811640, current state from region state map ={t,,1349484359011.dc01abf9cd7fd0ea256af4df02811640. state=OFFLINE, ts=1349484372508, server=null}

      this.gate.set(true);