fix for #128 - parentEvent is now nullable
fix for #128 - parentEvent is now nullable

thehyve/GSCF,PhenotypeFoundation/PhenotypeDatabase-legacy,NetherlandsMetabolomicsCentre/GSCF,NetherlandsMetabolomicsCentre/GSCF,PhenotypeFoundation/PhenotypeDatabase,PhenotypeFoundation/PhenotypeDatabase-legacy,TNO/PhenotypeDatabase,PhenotypeFoundation/PhenotypeDatabase-legacy,TNO/PhenotypeDatabase,PhenotypeFoundation/PhenotypeDatabase,thehyve/GSCF