Trackbar value pointer deprecation

[bgottula]

After upgrading to Mint 21.1, I started getting the following warning on starting capture:

[ WARN:0] global ./modules/highgui/src/window.cpp (703) createTrackbar UI/Trackbar(agc mode@Histogram): Using 'value' pointer is unsafe and deprecated. Use NULL as value pointer. To fetch trackbar value setup callback.
[ WARN:0] global ./modules/highgui/src/window.cpp (703) createTrackbar UI/Trackbar(gain@Histogram): Using 'value' pointer is unsafe and deprecated. Use NULL as value pointer. To fetch trackbar value setup callback.
[ WARN:0] global ./modules/highgui/src/window.cpp (703) createTrackbar UI/Trackbar(exposure time [us]@Histogram): Using 'value' pointer is unsafe and deprecated. Use NULL as value pointer. To fetch trackbar value setup callback.

Figure out how to deal with this (or suppress the warning).