The program crashes when G4ReactionDynamics::GenerateXandPt() is called with
vecLen == 0.  I don't understand the code well enough to know if vecLen=0 is an
invalid input, but most of the routine is protected against it.  However the
block:

      // DEBUGGING --> DumpFrames::DumpFrame(vec, vecLen);
      if( backwardParticlesLeft == 0)
      {
        backwardEnergy += targetParticle.GetMass()/GeV;
        targetParticle = *vec[0];
        --backwardCount;
        for( G4int j=0; j<(vecLen-1); ++j )*vec[j] = *vec[j+1];
        //G4ReactionProduct *temp = vec[vecLen];
        //delete temp;
        if( --vecLen == 0 )return false;  // all the secondaries have been
eliminated
        break;
      }

is not protected against vecLen == 0.  It crashes on the assignment of *vec[0]
to targetParticle.  If it did not crash here, it would get in more trouble when
it executed the --vecLen instruction and vecLen became negative, I think.

From a fairly naive analysis of the code, it looks to me like the routine
should return if vecLen==0 before the statement:

    while( forwardEnergy <= 0.0 )  // must eliminate a particle from the
forward side

Above this instruction there appear to be possibilities for vecLen to increase;
below it, particles can only be removed, and whenever --vecLen yields 0 after
this point the routine does in fact return.