Even after setting non-zero value of mag field it is ignored in particles
tracking in example N02.

After adding the call to SetDetectorField(this)
to the field manager in the ExN02MagneticField constructor,
it works ok:

ExN02MagneticField::ExN02MagneticField()
  : G4UniformMagField(G4ThreeVector())
{
  // need to be added
  GetGlobalFieldManager()->SetDetectorField(this);
  GetGlobalFieldManager()->CreateChordFinder(this);
}

ExN02MagneticField::ExN02MagneticField(G4ThreeVector fieldVector)
  : G4UniformMagField(fieldVector)
{
  // need to be added
  GetGlobalFieldManager()->SetDetectorField(this);
  GetGlobalFieldManager()->CreateChordFinder(this);
}