析构函数出现断点：请教一下在Vs2019编译器中为什么会出现断点问题，是哪里的内存分配出问题了嘛？

Question

#include<iostream>
#include<cstring>
using namespace std;
class String {
public:
    String(const char* c = NULL);//构造函数
    String(const String& s) : Cstr(new char[strlen(s.Cstr) + 1]) {//拷贝构造函数
        strcpy(Cstr, s.Cstr);
    }
    ~String() {//析构函数
        if (Cstr) delete[]Cstr;
    }
    int Length();//字符串长度
    char& operator [](int n);//下标运算符重载
    String operator +(const String& s);
    String& operator =(const String& s);//重载赋值运算符=//重载加法运算符
    String& operator +=(const String& s);//重载运算符+=
    bool operator < (const String& s);//重载运算符<
    bool operator > (const String& s);//重载运算符>
    bool operator == (const String& s);//重载运算符==
    friend ostream& operator <<(ostream& output, const String& s);//友元函数重载插入运算符
    //friend istream& operator >>(istream& intput, const String& s);//友元函数重载提取运算符
    void Screen_output();//屏幕输出
private:
    char* Cstr;
};

String::String(const char* c) {//构造函数 const char*c=NULL错误
    if (c == NULL) {
        Cstr = new char[1];
        *Cstr = '\0';//字符0
    }
    else {
        Cstr = new char[strlen(c) + 1];
        strcpy(Cstr, c);
    }
}

int String::Length() {
    int n = strlen(Cstr);
    return n;
}

char& String::operator [](int n) {
    return Cstr[n];
}


String& String::operator=(const String& s) {
    if (Cstr == s.Cstr)
        return *this;
    else if (NULL == this) {
        delete Cstr;
        Cstr = new char[strlen(s.Cstr) + 1];
        strcpy(Cstr, s.Cstr);
    }
    else {
        strcpy(Cstr, s.Cstr);
    }
    return *this;
}

String String:: operator +(const String& s) {
    char* c = new char[strlen(Cstr) + strlen(s.Cstr) + 1];
    c = strcat(Cstr, s.Cstr);
    return String(c);
}

String& String:: operator +=(const String& s) {
    strcat(Cstr, s.Cstr);
    return *this;
}

bool String::operator <(const String& s) {
    return strcmp(Cstr, s.Cstr) < 0;
}
bool String::operator >(const String& s) {
    return strcmp(Cstr, s.Cstr) > 0;
}
bool String::operator ==(const String& s) {
    return strcmp(Cstr, s.Cstr) == 0;
}
ostream& operator << (ostream& output, const String& s) {
    output << s.Cstr;
    return output;
}
//istream& operator >> (istream& intput, const String& s) {
    //intput >> s.Cstr;
    //if (intput)
    //return intput;
//}

void String::Screen_output() {
    String s1("Hello"), s2("word"), s3, s4;
    cout << "现有两个字符串：" << "s1=" << s1 << " s2=" << s2 <<endl;
    cout << "s1长度为：" << s1.Length() << endl;
    s3 = s1 + s2;
    cout << "s1+s2=" << s3<< endl;
    s1 += s2;
    cout << "s1+=s2后，s1=" << s1 << endl;
    s4 = s2;
    cout << "给s4赋值以后，s4=" << s4 << endl;
    s1[0] = 'h';
    cout << "改变s1首字符后，s1=" << s1 << endl;
    cout << "判断s2是否和s4相等，" << boolalpha <<(s2==s4)<< endl;
    cout << "判断s3>s1对错，" << boolalpha << (s3 > s1) << endl;
    cout << "判断s3<s1对错，" << boolalpha << (s3 < s1) << endl;
    
}
int main()
{
    String string;
    string.Screen_output();
    return 0;
}