Exception thrown: read access violation. **this** was nullptr. occurred - c++

Question

I am a novice at C++. I have started writing a rather large code on a project and I want to make a good base for that. I get the following error when running in Visual Studio:
Exception thrown: read access violation. this was nullptr. occurred
After searching similar questions, I could not find the solution for my code. The error is occurring in Driver.cpp at:
m_setup->SetEquation(...)
Any help is really appreciated. I also appreciate if you have suggestion to improve my code structure.
Main.h:
#ifndef FEM_AD1D_MAIN_H
#define FEM_AD1D_MAIN_H
namespace FEM_AD1D
{
class Driver;
Driver* m_driver;
}
#endif
Main.cpp:
#include "Main.h"
#include "Driver.h"
#include <iostream>
using namespace FEM_AD1D;
int main()
{
std::cout << "Hello World!\n";
m_driver->Run();
}
Driver.h:
#ifndef FEM_AD1D_DRIVER_H
#define FEM_AD1D_DRIVER_H
namespace FEM_AD1D
{
class Setup;
class Driver
{
public:
Driver(Setup* setup);
void Run();
void PreProc();
private:
Setup* m_setup;
const int m_nDim = 1;
const double m_aCoef = 1.0;
const double m_bCoef = 3.0;
const double m_cCoef = 0.0;
const double m_fCoef = 1.0;
const double m_xMin = 0.0;
const double m_xMax = 1.0;
const int m_nElem = 5;
const int m_elemType = 2;
const double m_meshStretch = 0.0;
const int m_nGaussPoint = 3;
};
}
#endif
Driver.cpp:
#include "Driver.h"
#include "Setup.h"
#include <iostream>
#include <array>
using namespace FEM_AD1D;
Driver::Driver(Setup* setup)
{
m_setup = setup;
//m_setup = new Setup();
}
void Driver::Run()
{
PreProc();
}
void Driver::PreProc()
{
std::cout << "Hello World 2!\n";
m_setup->SetEquation(m_nDim, m_aCoef, m_bCoef, m_cCoef, m_fCoef);
}
Setup.h:
#ifndef FEM_AD1D_SETUP_H
#define FEM_AD1D_SETUP_H
#include <vector>
namespace FEM_AD1D
{
class Setup
{
public:
Setup();
void SetEquation(int nDim, double aCoef, double bCoef, double cCoef, double fCoef);
private:
int m_nDim;
double m_aCoef;
double m_bCoef;
double m_cCoef;
double m_fCoef;
};
}
#endif
Setup.cpp:
#include "Setup.h"
#include "Driver.h"
#include <iostream>
#include <vector>
using namespace FEM_AD1D;
Setup::Setup() :
m_nDim(0),
m_aCoef(0.0),
m_bCoef(0.0),
m_cCoef(0.0),
m_fCoef(0.0)
{
}
void Setup::SetEquation(int nDim, double aCoef, double bCoef, double cCoef, double fCoef)
{
// Set the coefficients of the equation
std::cout << "Hello World 3!\n";
m_nDim = nDim;
m_aCoef = aCoef;
m_bCoef = bCoef;
m_cCoef = cCoef;
m_fCoef = fCoef;
}