I posted a bug report on May 1, 2016. This provides more information about that as follows
The original:
"When attempting to use color picker to sample color on screen, if the cursor is not completely within bounds of the screen, a crash occurs. This usually happens when sampling is set to some larger array of pixels. It probably happens even if set to 3 x 3 pixels but it is more difficult to be partially off screen on the level. However, with larger sampling arrays I have several times accidentally allowed the cursor to be outside of the screen bounds and a crash occurs every time.
This is a repeatable problem. Just put sampling on 31 x 31 or 51 x 51 and click when the cursor is partially off screen. A crash occurs every time.
John Murphy (a grateful user)"
I have found that if the color picker is off-canvas at the top or sides the problem does not happen. But when the color picker pointer is overlapping at the bottom of the canvas, where part of the pointer is on the canvas and part is below the canvas, paint.net crashes, for me every time I try it. The attached file shows a typical position of the color tool cursor when the crash occurs. The large cursor is to make it more visible.