following the rule of three but still compiler breaks inside destructor c++

Question

Hello here some of the main member function that constructs my own string class.

When compilers return the new object from friend MyString operator+ function it calls the destructor and then it breaks in the delete[] data;

This how looks part of the class:

class MyString
{
public:

    ...

//create a string from a null-terminated array
MyString(const char *cp):data(new char[std::strlen(cp)+1]), data_length(std::strlen(cp))
{
    std::copy(cp, cp + std::strlen(cp), stdext::checked_array_iterator<char*>(data, data_length));
    data[data_length] = '\0';
}       

    //copy constructor
    MyString(const MyString& str) 
        :data(new char[str.data_length]), 
        data_length(str.data_length)
    {
        std::copy(str.data, str.data + str.data_length, 
        stdext::checked_array_iterator<char*>(data, data_length));
        data[data_length] = '\0';
    }

    //assignment operator #1
    MyString& operator=(const char *cp)
    {
        if (cp != data)
        {
            data_length = std::strlen(cp);
            delete[] data;//delete the old data
            data = new char[std::strlen(cp) + 1];//create new data array
            std::copy(cp, cp + std::strlen(cp), stdext::checked_array_iterator<char*>(data, data_length));
            data[data_length] = '\0';
        }

        return *this;
    }


    //assignment operator #2 with self-assignment check
    MyString& operator=(const MyString& str)
    {
        if (&str != this)
        {
            data_length = std::strlen(str.data);
            delete[] data;//delete the old data
            data = new char[str.size() + 1];//create new data array
            std::copy(str.data, str.data + str.size(), stdext::checked_array_iterator<char*>(data, data_length));
            data[data_length] = '\0';
        }

        return *this;
    }


    //destructor
    ~MyString()
    {
        //free the array
        if (data != nullptr)
        {
            delete[] data;
        }           
    };


    MyString& operator+=(MyString& s)
    {
        std::copy(s.data, s.data + data_length, 
            stdext::checked_array_iterator<char*>(data+data_length, data_length));
        data[data_length + s.data_length] = '\0';
        data_length += s.data_length;
        return *this;
    }

    friend MyString operator+(MyString& s1, MyString& s2)
    {
        MyString str = s1;
        str += s2;
        return str;
    }


private:
    size_t data_length;
    char* data;

};


int main()
{

    MyString mystr1 = "hello";  
    MyString s_con = mystr1 + mystr1;

    return 0;
}