Error when get color from pixel in visual studio C++

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This is function to get the color from a given Mat image,

string getColorFromPx(Mat *src, uint16_t x, uint16_t y){
Point pt = Point(x, y);
//Mat img = (*src).clone();
Vec3b& levelPxImg = imgHSV.at<Vec3b>(y, x);
if ((levelPxImg[0] != NULL) && (levelPxImg[1] != NULL) && (levelPxImg[2] != NULL)){
    //H(openCV) / 2 = H (HSV_Real);
    if (levelPxImg[0] > 170 && levelPxImg[0] < 180)
        return "R";
    else if (levelPxImg[0] > 107 && levelPxImg[0] < 122)
        return "B";
    else
        return "xx";
}

}

the program can work for a period time after that it notify an error as below:

" First-chance exception at 0x00007FF64C1C30A3 in OpenCVex1.exe: 0xC0000005: Access violation reading location 0x0000000000000018. If there is a handler for this exception, the program may be safely continued. "

and current breakpoint is halted at return line as below:

int compare(const _Elem *_Ptr) const {// compare [0, _Mysize) with [_Ptr, <null>)

_DEBUG_POINTER(_Ptr);

-------> return (compare(0, this->_Mysize, _Ptr, _Traits::length(_Ptr))); }

(this function locate in VS/include/xstring file).

Please take a look and help me to show problem.