Segmentation fault when calling the pointer memeber in based class

In my project, there is a pointer to the object of a class, in which one of the class member is a pointer to struct. I want to access to the pointer to struct and reset its value. But when I tried calling it, even just printing its current value, the segmentation fault would be throwm off.

I found the error occured when I calling the pointer, even just calling it and printing its value.

Here is part of my code.

In A.h header file:

#ifndef IMU_TOOL_H
#define IMU_TOOL_H

#include <fstream>
#include <iostream>

#include <Eigen/Core>
#include <Eigen/Geometry>
#include <Eigen/Dense>
......
......
#include <gtsam/navigation/ImuFactor.h>
......
......

using namespace gtsam;
using namespace std;

namespace Mobile_Sensing
{
class IMUHelper 
{
public:
    EIGEN_MAKE_ALIGNED_OPERATOR_NEW

    IMUHelper()
    {
        ......
        ......

    priorVelocity_ = Vector3(0, 0, 0);

    lastTimestamp_ = 0;

        ......
        ......
    }


public:        
    Vector3 priorVelocity_;
    long long lastTimestamp_;
    ......
    ......

};

class Factor_Graph_Optimization
{
public:
    EIGEN_MAKE_ALIGNED_OPERATOR_NEW

public:
    Factor_Graph_Optimization();

    void backendInitialization(Scan* scan, SPANPose* imu);

    inline void setVelocityPrior(const Vector3& priorVelocity) { 
        imuTool_->priorVelocity_  = priorVelocity;}

    inline void setStartTimestamp(const long long& startTime) { 
    imuTool_->lastTimestamp_ = startTime; }

public:


    IMUHelper* imuTool_;

    ......
    ......

};

}

#endif

In the A.cpp source file

#include <A.h>

namespace Mobile_Sensing 
{

Factor_Graph_Optimization::Factor_Graph_Optimization()
{   
    imuTool_ = new IMUHelper();
    imuTool_->initialization();

    ......
    ......
}

void Factor_Graph_Optimization::backend_initialization(Scan* scan, SPANPose* imu)
{
    // if (NULL==imuTool_) // ERROR OCCURED HERE!!!
        // std::cout << "ERROR: empty imuTool pointer!!" << std::endl;

    // set velocity prior
    Vector3 priorVelocity(imu->vf_, imu->vl_, imu->vu_);
    // set_velocity_prior(priorVelocity); // ERROR OCCURED HERE!!!

    // set start timestamp
    setStarttimestamp(imu->timestamp_); // ERROR OCCURED HERE!!!

    ......
    ......
}


}

The segmentation error occured when I calling the pointer. I don't why and how to solve this problem. Please help!!!!!!!