Sometimes crashes on agent (areOnMaster)

[kdeenanauth]

Sometimes we receive an error on a Jenkins agent

java.lang.IllegalStateException: Jenkins.instance is missing. Read the documentation of Jenkins.getInstanceOrNull to see what you are doing wrong.
	at jenkins.model.Jenkins.get(Jenkins.java:772)
	at hudson.model.Hudson.getInstance(Hudson.java:77)
	at com.google.jenkins.plugins.source.GoogleRobotUsernamePassword.areOnMaster(GoogleRobotUsernamePassword.java:146)
	at com.google.jenkins.plugins.source.GoogleRobotUsernamePassword.readObject(GoogleRobotUsernamePassword.java:180)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:1058)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2136)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2027)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1535)
	at java.io.ObjectInputStream.readArray(ObjectInputStream.java:1933)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1529)
	at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2245)
	at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2169)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2027)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1535)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:422)
	at hudson.remoting.UserRequest.deserialize(UserRequest.java:245)
	at hudson.remoting.UserRequest.perform(UserRequest.java:159)

It looks like we shouldn't be calling Hudson.getInstance to determine whether we're running on a Jenkins agent. There may be other classes that shouldn't be called/remoted from this plugin.

[viglesiasce]

Fixed in #9