I enabled Xvfb in a job, configured both the master and the slave to create a default Xvfb config and properly locate the binary on the slave. Every time I run the job the result is the following backtrace:

Xvfb startingFATAL: null
java.lang.NullPointerException
at com.cloudbees.jenkins.plugins.customtools.CustomToolInstallWrapper$2.toEnvVars(CustomToolInstallWrapper.java:142)
at com.cloudbees.jenkins.plugins.customtools.CustomToolInstallWrapper$2.launch(CustomToolInstallWrapper.java:131)
at hudson.Launcher$ProcStarter.start(Launcher.java:338)
at org.jenkinsci.plugins.xvfb.XvfbBuildWrapper.decorateLauncher(XvfbBuildWrapper.java:191)
at hudson.model.AbstractBuild$AbstractBuildExecution.createLauncher(AbstractBuild.java:549)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:475)
at hudson.model.Run.execute(Run.java:1502)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:236)