When I upgrade to version 1.150 of the sauce-on-demand plugin, I got this error and it failed to install.
Re-installing version 1.149 of the plugin worked.

{{Apr 03, 2016 1:33:16 PM jenkins.InitReactorRunner$1 onTaskFailed
SEVERE: Failed Loading plugin sauce-ondemand
java.io.IOException: Failed to initialize
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:441)
at hudson.PluginManager$2$1$1.run(PluginManager.java:384)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
at jenkins.model.Jenkins$8.runTask(Jenkins.java:924)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.io.IOException: Unable to read /var/lib/jenkins/sauce-ondemand.xml
at hudson.XmlFile.unmarshal(XmlFile.java:165)
at hudson.Plugin.load(Plugin.java:264)
at hudson.plugins.sauce_ondemand.PluginImpl.start(PluginImpl.java:95)
at hudson.ClassicPluginStrategy.startPlugin(ClassicPluginStrategy.java:449)
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:438)
... 9 more
Caused by: com.thoughtworks.xstream.converters.ConversionException: null : null
---- Debugging information ----
cause-exception : java.lang.NullPointerException
cause-message : null
class : hudson.plugins.sauce_ondemand.PluginImpl
required-type : hudson.plugins.sauce_ondemand.PluginImpl
converter-type : hudson.util.RobustReflectionConverter
path : /hudson.plugins.sauce_ondemand.PluginImpl
line number : 12
version : not available
-------------------------------
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:79)
at com.thoughtworks.xstream.core.AbstractReferenceUnmarshaller.convert(AbstractReferenceUnmarshaller.java:65)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:66)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convertAnother(TreeUnmarshaller.java:50)
at com.thoughtworks.xstream.core.TreeUnmarshaller.start(TreeUnmarshaller.java:134)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.unmarshal(AbstractTreeMarshallingStrategy.java:32)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1189)
at hudson.util.XStream2.unmarshal(XStream2.java:114)
at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1173)
at hudson.XmlFile.unmarshal(XmlFile.java:163)
... 13 more
Caused by: java.lang.NullPointerException
at hudson.diagnosis.OldDataMonitor.report(OldDataMonitor.java:220)
at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:330)
at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:229)
at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
... 22 more
}}