PHP Fatal error when importing SKOS

[jesus san miguel]

Importing a previously exported XML causes PHP error:

2020-09-09 19:44:59 > New ability: arFindingAidJob
2020-09-09 19:44:59 > New ability: arInheritRightsJob
2020-09-09 19:44:59 > New ability: arUpdateEsActorRelationsJob
2020-09-09 19:44:59 > New ability: arCalculateDescendantDatesJob
2020-09-09 19:44:59 > New ability: arObjectMoveJob
2020-09-09 19:44:59 > New ability: arInformationObjectCsvExportJob
2020-09-09 19:44:59 > New ability: qtSwordPluginWorker
2020-09-09 19:44:59 > New ability: arUpdatePublicationStatusJob
2020-09-09 19:44:59 > New ability: arFileImportJob
2020-09-09 19:44:59 > New ability: arInformationObjectXmlExportJob
2020-09-09 19:44:59 > New ability: arXmlExportSingleFileJob
2020-09-09 19:44:59 > New ability: arGenerateReportJob
2020-09-09 19:44:59 > New ability: arActorCsvExportJob
2020-09-09 19:44:59 > New ability: arActorXmlExportJob
2020-09-09 19:44:59 > New ability: arRepositoryCsvExportJob
2020-09-09 19:44:59 > New ability: arUpdateEsIoDocumentsJob
2020-09-09 19:44:59 > New ability: arPhysicalObjectCsvHoldingsReportJob
2020-09-09 19:44:59 > Running worker...
2020-09-09 19:44:59 > PID 15
2020-09-09 19:44:59 > Job started.
2020-09-09 19:44:59 > Importing SKOS file: agua.xml.
2020-09-09 19:44:59 > Type of scheme: file
2020-09-09 19:44:59 > Taxonomía: Materias
2020-09-09 19:44:59 > Term ID: 1
2020-09-09 19:44:59 > El grafo contiene 3 conceptos.
PHP Fatal error:  Uncaught Error: Call to a member function getValue() on null in /atom/src/plugins/sfSkosPlugin/lib/sfSkosPlugin.class.php:156
Stack trace:
#0 /atom/src/lib/job/arFileImportJob.class.php(74): sfSkosPlugin->importGraph()
#1 /atom/src/lib/job/arBaseJob.class.php(119): arFileImportJob->runJob(Array)
#2 /atom/src/vendor/net_gearman/Net/Gearman/Worker.php(351): arBaseJob->run(Array)
#3 /atom/src/vendor/net_gearman/Net/Gearman/Worker.php(247): Net_Gearman_Worker->doWork(Resource id #1879)
#4 /atom/src/lib/task/jobs/jobWorkerTask.class.php(123): Net_Gearman_Worker->beginWork(Object(Closure))
#5 /atom/src/vendor/symfony/lib/task/sfBaseTask.class.php(68): jobWorkerTask->execute(Array, Array)
#6 /atom/src/vendor/symfony/lib/task/sfTask.class.php(97): sfBaseTask->doRun(Object(sfCommandManager), NULL)
#7 /atom/src/vendor/symfony/lib/command/sfSymfonyCommandApplication.class.php(76): sfTask->runFromCLI(Object(sfCommandManager), NULL)
#8 /atom/src/vendor/symfony/lib/command/cli.php(20): sfSymfonyCommandApplication->run()
#9 in /atom/src/plugins/sfSkosPlugin/lib/sfSkosPlugin.class.php on line 156

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