CompoundStitcher.getRoot needs safe call, in case of new nodes during restitching

[southalln]

http://localhost:8080/api/stitches/latest/DB00117/@testupdate
{"statusMessage":"Error stitching: 66793\n0","status":"failure","request":{"node":368958,"nodeId":"DB00117","value":"30KYC7MIAI","jsonPath":"$['properties'][?(@['key']=='unii' )]['value']","oldValue":"4QD397987E","nodeSource":"DrugBank, December 2018","operation":"replace","stitches":"DB00117"}}
DB00117 DB00128 {u'status': u'failure', u'request': {u'node': 368958, u'nodeId': u'DB00117', u'value': u'30KYC7MIAI', u'jsonPath': u"$['properties'][?(@['key']=='unii' )]['value']", u'oldValue': u'4QD397987E', u'nodeSource': u'DrugBank, December 2018', u'operation': u'replace', u'stitches': u'DB00117'}, u'statusMessage': u'Error stitching: 66793\n0'}