[regression] NPE when running Checkstyle

[INFO] An error has occurred in Checkstyle report generation.

[INFO] ------------------------------------------------------------------------
[INFO] Trace
org.apache.maven.lifecycle.LifecycleExecutionException: An error has occurred in Checkstyle report generation.
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:524)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.forkProjectLifecycle(DefaultLifecycleExecutor.java:862)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.forkLifecycle(DefaultLifecycleExecutor.java:699)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:470)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:440)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:419)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:271)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:238)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:146)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:303)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:123)
        at org.apache.maven.embedder.MavenEmbedder.execute(MavenEmbedder.java:907)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:369)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:585)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:408)
        at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:351)
Caused by: org.apache.maven.plugin.MojoExecutionException: An error has occurred in Checkstyle report generation.
        at org.apache.maven.reporting.AbstractMavenReport.execute(AbstractMavenReport.java:79)
        at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:598)
        at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:499)
        ... 20 more
Caused by: java.lang.NullPointerException
        at java.io.Reader.<init>(Reader.java:61)
        at java.io.InputStreamReader.<init>(InputStreamReader.java:55)
        at org.codehaus.plexus.util.IOUtil.copy(IOUtil.java:248)
        at org.codehaus.plexus.util.IOUtil.toString(IOUtil.java:307)
        at org.codehaus.plexus.util.IOUtil.toString(IOUtil.java:295)
        at org.apache.maven.reporting.AbstractMavenReport.getSiteDescriptor(AbstractMavenReport.java:134)
        at org.apache.maven.reporting.AbstractMavenReport.execute(AbstractMavenReport.java:70)
        ... 22 more

  <build>
    <plugins>
      <plugin>
        <!-- Apply the Checkstyle configurations defined in the top level pom.xml file -->
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-checkstyle-plugin</artifactId>
        <dependencies>
          <dependency>
            <groupId>com.xpn.xwiki</groupId>
            <artifactId>xwiki-build-verifications</artifactId>
            <version>1.0-SNAPSHOT</version>
          </dependency>
        </dependencies>
        <configuration>
          <includes>
              **/Api.java,
              **/xmlrpc/Attachment.java,
              **/xmlrpc/SpaceSummary.java,
              **/ViewAction.java,
              **/ZipExplorer*.java,
              **/content/**/*.java,
              **/XWikiMessageTool.java
          </includes>
        </configuration>
      </plugin>
    </plugins>
  </build>