I ofted read the following stack trance in my Jenkins log. Despite this, the plugin seems to work properly, correctly displaying disk usage.

Nov 13, 2014 3:10:02 PM hudson.model.listeners.ItemListener$1 run
WARNING: failed to send event to listener of class hudson.plugins.disk_usage.DiskUsageItemListener
java.lang.NullPointerException
at hudson.plugins.disk_usage.DiskUsageUtil.loadData(DiskUsageUtil.java:84)
at hudson.plugins.disk_usage.DiskUsageUtil.addProperty(DiskUsageUtil.java:64)
at hudson.plugins.disk_usage.DiskUsageItemListener.onUpdated(DiskUsageItemListener.java:40)
at hudson.model.listeners.ItemListener$4.apply(ItemListener.java:201)
at hudson.model.listeners.ItemListener$4.apply(ItemListener.java:199)
at hudson.model.listeners.ItemListener$1.run(ItemListener.java:166)
at hudson.security.ACL.impersonate(ACL.java:144)
at hudson.model.listeners.ItemListener.forAll(ItemListener.java:174)
at hudson.model.listeners.ItemListener.fireOnUpdated(ItemListener.java:199)
at hudson.model.AbstractProject.makeDisabled(AbstractProject.java:706)
at hudson.model.AbstractProject.enable(AbstractProject.java:725)
at com.github.mjdetullio.jenkins.plugins.multibranch.FreeStyleMultiBranchProject._syncBranches(FreeStyleMultiBranchProject.java:1115)
at com.github.mjdetullio.jenkins.plugins.multibranch.FreeStyleMultiBranchProject.syncBranches(FreeStyleMultiBranchProject.java:1020)
at com.github.mjdetullio.jenkins.plugins.multibranch.SyncBranchesTrigger.run(SyncBranchesTrigger.java:98)
at hudson.triggers.Trigger.checkTriggers(Trigger.java:266)
at hudson.triggers.Trigger$Cron.doRun(Trigger.java:214)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:178)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:744)