Steps I used to see the issue:

• installed Git Client plugin 1.1.1 and restarted Jenkins
• downloaded and installed Git Plugin 2.0 beta and restarted Jenkins
• executed several successful builds without changing configurations
• confirmed those builds were correctly responding to build hooks
• use configuration page to delete git as executable, add jgit
• submitted another change to git repository, watched hooks activate

No build from Jenkins, and a null pointer exception is in the log file.

SEVERE: Failed to record SCM polling for hudson.model.FreeStyleProject@21d1ff77[p4-port-linux-Merge]
java.lang.NullPointerException
at org.jenkinsci.plugins.gitclient.trilead.TrileadSessionFactory.getSession(TrileadSessionFactory.java:34)
at org.eclipse.jgit.transport.SshTransport.getSession(SshTransport.java:121)
at org.eclipse.jgit.transport.TransportGitSsh$SshFetchConnection.<init>(TransportGitSsh.java:248)
at org.eclipse.jgit.transport.TransportGitSsh.openFetch(TransportGitSsh.java:147)
at org.eclipse.jgit.transport.FetchProcess.executeImp(FetchProcess.java:136)
at org.eclipse.jgit.transport.FetchProcess.execute(FetchProcess.java:122)
at org.eclipse.jgit.transport.Transport.fetch(Transport.java:1105)
at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:128)
at org.jenkinsci.plugins.gitclient.JGitAPIImpl.fetch(JGitAPIImpl.java:299)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:275)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:256)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:215)
at hudson.remoting.UserRequest.perform(UserRequest.java:118)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)