I'm getting the following in my log file when jenkins starts up, after updating to 2.1 and the latest selenium plugin:

A thread (TCP agent connection handler #1 with /IP_OF_SLAVE_MACHINE:52178/67) died unexpectedly due to an uncaught exception, this may leave your Jenkins in a bad way and is usually indicative of a bug in the code.
java.lang.NullPointerException
at hudson.plugins.selenium.callables.SeleniumCallable.invoke(SeleniumCallable.java:112)
at hudson.plugins.selenium.callables.SeleniumCallable.invoke(SeleniumCallable.java:24)
at hudson.FilePath$FileCallableWrapper.call(FilePath.java:2719)
at hudson.remoting.UserRequest.perform(UserRequest.java:120)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at hudson.remoting.Engine$1$1.run(Engine.java:62)
at java.lang.Thread.run(Unknown Source)
at ......remote call to SLAVE_MACHINE_NAME(Native Method)
at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1416)
at hudson.remoting.UserResponse.retrieve(UserRequest.java:220)
at hudson.remoting.Channel.call(Channel.java:781)
at hudson.FilePath.act(FilePath.java:979)
at hudson.FilePath.act(FilePath.java:968)
at hudson.plugins.selenium.process.SeleniumJarRunner.start(SeleniumJarRunner.java:42)
at hudson.plugins.selenium.configuration.global.SeleniumGlobalConfiguration.start(SeleniumGlobalConfiguration.java:47)
at hudson.plugins.selenium.PluginImpl.startSeleniumNode(PluginImpl.java:485)
at hudson.plugins.selenium.ComputerListenerImpl.onOnline(ComputerListenerImpl.java:26)
at hudson.slaves.SlaveComputer.setChannel(SlaveComputer.java:573)
at jenkins.slaves.JnlpSlaveAgentProtocol$Handler.jnlpConnect(JnlpSlaveAgentProtocol.java:126)
at jenkins.slaves.DefaultJnlpSlaveReceiver.handle(DefaultJnlpSlaveReceiver.java:69)
at jenkins.slaves.JnlpSlaveAgentProtocol2$Handler2.run(JnlpSlaveAgentProtocol2.java:59)
at jenkins.slaves.JnlpSlaveAgentProtocol2.handle(JnlpSlaveAgentProtocol2.java:31)
at hudson.TcpSlaveAgentListener$ConnectionHandler.run(TcpSlaveAgentListener.java:181)

I've tested this with master access control both enabled and disabled, I get the same error in the log file.