Verified in code of master branch, IMHO relevant to most current releases.

According to the documentation of createTrackbar the third parameter to the function is an "Optional pointer to an integer". I read that as "you can pass a nullptr here".

Some HighGUI backends handle this scenario correctly:

• Win32 -- https://github.com/opencv/opencv/blame/master/modules/highgui/src/window_w32.cpp#L2368
• GTK -- https://github.com/opencv/opencv/blob/master/modules/highgui/src/window_gtk.cpp#L1421
• Cocoa -- https://github.com/opencv/opencv/blob/master/modules/highgui/src/window_cocoa.mm#L949

Some consider it an error:

• Qt -- https://github.com/opencv/opencv/blob/master/modules/highgui/src/window_QT.cpp#L1222 (invoked from https://github.com/opencv/opencv/blob/master/modules/highgui/src/window_QT.cpp#L605)

And some apparently dereference it without even checking:

• WINRT -- https://github.com/opencv/opencv/blob/master/modules/highgui/src/window_winrt_bridge.cpp#L274 (invoked from https://github.com/opencv/opencv/blob/master/modules/highgui/src/window_winrt.cpp#L115)

Code that should reproduce this (works on Windows, don't have means to test other backends):

#include <opencv2/opencv.hpp>

void on_trackbar(int pos, void* userdata)
{
}

int main()
{
    auto window_name("Test Window");
    cv::namedWindow(window_name);

    cv::createTrackbar("Trackbar", window_name, nullptr, 100, on_trackbar);

    cv::Mat temp = cv::Mat::zeros(256, 256, CV_8UC1);
    cv::imshow(window_name, temp);

    cv::waitKey();
    return 0;
}

Reference: https://stackoverflow.com/q/67690257/3962537