Possible nullpointer when computing XAR extension changes

Caused by: java.lang.NullPointerException
	at org.xwiki.extension.xar.internal.script.SafeDiffXarJobStatus.getDocumentDiffs(SafeDiffXarJobStatus.java:71)
	at sun.reflect.GeneratedMethodAccessor771.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.apache.velocity.runtime.parser.node.PropertyExecutor.execute(PropertyExecutor.java:142)
	at org.apache.velocity.util.introspection.UberspectImpl$VelGetterImpl.invoke(UberspectImpl.java:539)
	at org.apache.velocity.runtime.parser.node.ASTIdentifier.execute(ASTIdentifier.java:198)
	... 222 more