I get the following traceback when running a job with configured xvfb plugin and no node labels:

FATAL: null
java.lang.NullPointerException
at org.jenkinsci.plugins.xvfb.XvfbBuildWrapper.setUp(XvfbBuildWrapper.java:568)
at hudson.model.Build$BuildExecution.doRun(Build.java:153)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:518)
at hudson.model.Run.execute(Run.java:1706)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:231)

The line reads:

if (assignedLabels != null || assignedLabels.trim().isEmpty()) {

shouldn't that be && ?

A workaround is adding a label (i.e. master)