Iteration itr segmentation fault

Is there any error in the follwing core. When running, there is a segmentation fault.

G4THitsMap* evtMap =

std::map<G4int,G4double*>::iterator itr;
G4int copyNb= -1;
G4double edep= 0.0 ;

for (itr = evtMap->GetMap()->begin(); itr != evtMap->GetMap()->end(); itr++) {
copyNb = (itr->first);
edep = *(itr->second);
G4cout<<“copyNb”<<copyNb<<"; edep/keV"<<edep/keV<<G4endl;

G4double let = edep/Tsv*keV/nm;

G4cout << " target" << copyNb << ": " << edep/keV << " keV “<<” LET: "<<let<<“keV/nm”<<G4endl;

The evtMap is NULL, so it will happend that core dumped. But I have no idea that why it will NULL, why there is no hit in SV. I cannot access any data in the ProcessHits.