This is probably related to JENKINS-17877. It works with JUnit but not with Custom Tool.

[xUnit] [INFO] - Starting to record.
[xUnit] [INFO] - Processing Custom Tool
ERROR: Publisher org.jenkinsci.plugins.xunit.XUnitPublisher aborted due to exception
java.lang.NullPointerException
at com.thalesgroup.dtkit.metrics.hudson.api.type.MetricsType.isSkipNoTestFiles(MetricsType.java:80)
at org.jenkinsci.plugins.xunit.XUnitPublisher.getXUnitToolInfoObject(XUnitPublisher.java:211)
at org.jenkinsci.plugins.xunit.XUnitPublisher.performTests(XUnitPublisher.java:182)
at org.jenkinsci.plugins.xunit.XUnitPublisher.performXUnit(XUnitPublisher.java:119)
at org.jenkinsci.plugins.xunit.XUnitPublisher.perform(XUnitPublisher.java:93)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:804)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:776)
at com.cloudbees.plugins.flow.FlowRun$RunnerImpl.post2(FlowRun.java:210)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:726)
at hudson.model.Run.execute(Run.java:1600)
at hudson.model.Run.run(Run.java:1521)
at com.cloudbees.plugins.flow.FlowRun.run(FlowRun.java:174)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:241)
at hudson.model.OneOffExecutor.run(OneOffExecutor.java:66)