NPE in DigestComputer event listener when event has no source document

10:32:30,862 ERROR [EventServiceImpl] Error during sync listener execution
java.lang.NullPointerException
at org.nuxeo.ecm.platform.filemanager.core.listener.DigestComputer.handleEvent(DigestComputer.java:133)
at org.nuxeo.ecm.core.event.impl.EventServiceImpl.fireEvent(EventServiceImpl.java:123)
at org.nuxeo.ecm.core.api.AbstractSession.fireEvent(AbstractSession.java:330)
at org.nuxeo.ecm.core.api.AbstractSession.notifyEvent(AbstractSession.java:378)
at org.nuxeo.ecm.core.api.AbstractSession.save(AbstractSession.java:1643)
at org.nuxeo.ecm.core.api.ejb.DocumentManagerBean.save(DocumentManagerBean.java:285)