我的C++代码给出了代码中没有的错误.虫子是什么

编译此代码时，会出现如下所示的运行时错误。但它并不能说明我的代码中哪一行有错误。

调试断言失败

程序：C:\Windows\system32\MSVCP110D.dll
文件：c:\program files（x86）\microsoft visual studio 11.0\vc\include\xstring
线路：1143

表达式：无效的空指针

有关程序如何导致断言失败的信息，请参阅有关断言的Visual C++文档

（按"重试"以调试应用程序。）

下面是我的C++代码。由一个基类Vehicle和另一个从基类公开继承的派生类Car组成。

class Vehicle {
private:
    string VehicleNo, color;
public:
    Vehicle():VehicleNo(NULL),color(NULL){};
    string getVehicleNo(){return VehicleNo;}
    string getColor(){return color;}
    void setVehicleNo(){
        //getline(cin,VehicleNo);
        cin>>VehicleNo;
    }
    void setVehicleColor(){cin>>color;}
};

class Car: public Vehicle {
private:
    int distance;
public:
    void setDistance(int x){distance=x;}
    void setCarNo(){setVehicleNo();}
    void setCarColor(){setVehicleColor();}
    int calculateFare(int x){return 5*x;};

    void displayInformation()
    {
        cout<<"Your Car Number is: "<<getVehicleNo()<<endl;
        cout<<"The color of your Car is: "<<getColor()<<endl;
        cout<<"Total fare which you have to pay: "<<calculateFare(distance);
    }

};
int main()
{
    Car c1;
    int distance;
    char choice;
    cout<<"Enter car number: ";
    c1.setCarNo();
    cout<<"nEnter Car Color: ";
    c1.setCarColor();
    cout<<"nHow long would you like to go? Enter distance in kilometers: ";
    cin>>distance;
    c1.setDistance(distance);
    cout<<"n----------------------------------n";
    c1.displayInformation();
    cout<<"n----------------------------------n";
    cout<<"nDo you want to calculate Fare of different distance (y/Y for yes and another character for No?  ";
    cin>>choice;
    do{
        cout<<"nHow long would you like to go? Enter distance in Kilometers: ";
        cin>>distance;
        cout<<"n----------------------------------n";
        c1.setDistance(distance);
        c1.displayInformation();
        cout<<"nDo you want to calculate Fare of different distance (y/Y for yes and another character for No?  ";
        cin>>choice;
    } 
    while(choice=='y' || choice=='Y');
}