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Why does findcontours crushes the program in Qt?

I wrote a program that finds contours-rectangles- in a video stream from web-cam. I tried it with visual studio 2015 and worked well. But, when i try it on Qt - to design a GUI- it crashes at findcontours with no error explanation, only gives me

The program has unexpectedly finished. C:\Users\Amr\Documents\build-untitled10-Desktop_Qt_5_8_0_MSVC2015_64bit-Release\release\untitled10.exe crashed.

i have searched a lot,but unfortunately didn't find any thing helpful.

project.h file

#include <iostream>
#include <QMainWindow>
#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <QDebug>
#include<QtCore>
using namespace std;
using namespace cv;

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
private:
    Ui::MainWindow *ui;
    cv::VideoCapture vid;
    cv::Mat original;
    cv::Mat gray;
    cv::Mat dst ;

    QImage qoriginal;
    QImage qgray;
    QImage qdst;

    QTimer *timer;

    vector<vector<Point>>contours;
    vector<Vec4i> hierarchy;

public slots:
    void camerashow();

private slots:
    void on_pushButton_clicked();

};

main window

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    vid.open(0);
    if (!vid.isOpened()){qDebug()<<"error";}
    timer = new QTimer(this);
    connect (timer,SIGNAL(timeout()),this,SLOT(camerashow()));

}

and the camerashow function

vid.read(original); // read camera feed

    cvtColor(original,gray,CV_BGR2GRAY); // convert to a format that qt understand

    blur(gray, gray, Size(3, 3));
    Canny(gray, gray, 120, 255,3);

    findContours(gray, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE);

        for (int i = 0; i < contours.size(); i++)
        {

       drawContours(original, contours, i, Scalar(0,0,255), 2, 8, hierarchy);             
      }
   cv::cvtColor(original,original,CV_BGR2RGB);   
   QImage qoriginal((uchar*)original.data,original.cols,original.rows,original.step,QImage::Format_RGB888);
   QImage qgray((uchar*)gray.data,gray.cols,gray.rows,gray.step,QImage::Format_Grayscale8);

        // equivilant to imshow
    ui->label ->setPixmap(QPixmap::fromImage(qoriginal));
    ui->label_2 ->setPixmap(QPixmap::fromImage(qgray));

}

when i comment the for loop, it also crushes. when commenting findcontour, it works and shows original and canny outputs.

i'm using OpenCV 2.4.9 visual studio 2015 Qt creator 5.8.0 and working in release mode

Edit: when i run it in debug mode, it works with no error but gives me

Debug assertion failed "(_ptr_user&(_big_allocation_alignment -1)==0" &&0

Why does findcontours crushes crash the program in Qt?

The program has unexpectedly finished. C:\Users\Amr\Documents\build-untitled10-Desktop_Qt_5_8_0_MSVC2015_64bit-Release\release\untitled10.exe crashed.