Following the plugin tutorial (https://wiki.jenkins-ci.org/display/JENKINS/Plugin+tutorial).

• mvn -cpu hpi:create (groupId org.example, artifactId simple-plugin)
• cd simple-plugin
• Changed parent element in POM to
  <groupId>org.jenkins-ci.plugins</groupId>
  <artifactId>plugin</artifactId>
  <version>1.427</version>
• mvn clean install
• mvn hpi:run -Djetty.port=8090
  Trying to bring up Web UI with http://localhost:8090/ gives
  Caused by: java.lang.NullPointerException
  at org.mortbay.io.Portable.getBytes(Portable.java:46)
  at org.mortbay.io.ByteArrayBuffer.<init>(ByteArrayBuffer.java:66)
  at org.mortbay.io.ByteArrayBuffer$CaseInsensitive.<init>(ByteArrayBuffer.java:115)
  at org.mortbay.io.BufferCache$CachedBuffer.<init>(BufferCache.java:118)
  at org.mortbay.io.BufferCache.lookup(BufferCache.java:91)
  at org.mortbay.jetty.HttpFields.add(HttpFields.java:604)
  at org.mortbay.jetty.Response.addHeader(Response.java:445)
  at javax.servlet.http.HttpServletResponseWrapper.addHeader(HttpServletResponseWrapper.java:168)
  at org.kohsuke.stapler.jelly.HeaderTag.doTag(HeaderTag.java:59)
  at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:270)
  (Full output of hpi:run is attached.)