Add a magnetic field

Hello @bmorgan thank you.
I fixed it and now the simulation compiles…but it dies without runnning

**************************************************************
 Geant4 version Name: geant4-10-07-patch-01 [MT]   (5-February-2021)
  << in Multi-threaded mode >>
                       Copyright : Geant4 Collaboration
                      References : NIM A 506 (2003), 250-303
                                 : IEEE-TNS 53 (2006), 270-278
                                 : NIM A 835 (2016), 186-225
                             WWW : http://geant4.org/
**************************************************************

<<< Reference Physics List QBBC
Using Root
Visualization Manager instantiating with verbosity "warnings (3)"...
Visualization Manager initialising...
Registering graphics systems...

You have successfully registered the following graphics systems.
Registered graphics systems are:
  ASCIITree (ATree)
  DAWNFILE (DAWNFILE)
  G4HepRep (HepRepXML)
  G4HepRepFile (HepRepFile)
  RayTracer (RayTracer)
  VRML1FILE (VRML1FILE)
  VRML2FILE (VRML2FILE)
  gMocrenFile (gMocrenFile)
  OpenGLImmediateQt (OGLIQt, OGLI)
  OpenGLStoredQt (OGLSQt, OGL, OGLS)
  Qt3D (Qt3D)

Registering model factories...

You have successfully registered the following model factories.
Registered model factories:
  generic
  drawByAttribute
  drawByCharge
  drawByOriginVolume
  drawByParticleID
  drawByEncounteredVolume

Registered models:
  None

Registered filter factories:
  attributeFilter
  chargeFilter
  originVolumeFilter
  particleFilter
  encounteredVolumeFilter

Registered filters:
  None

You have successfully registered the following user vis actions.
Run Duration User Vis Actions: none
End of Event User Vis Actions: none
End of Run User Vis Actions: none

Some /vis commands (optionally) take a string to specify colour.
"/vis/list" to see available colours.
Checking overlaps for volume Magnet (G4Box) ... OK!
Checking overlaps for volume Regenerator (G4Tubs) ... OK!
Checking overlaps for volume Target1 (G4Tubs) ... OK!
Checking overlaps for volume Target2 (G4Tubs) ... OK!

C:\B1\B1-build\Release>