Crash with system Color Picker

An user of my app RealCADD can crash it on Big Sur when using the Color Picker to choose and store a new custom color.
But not when he uses the color picker first in another app.
And I can’t reproduce the bug nor on Catalina or on Big Sur.

The bug report says :
Application Specific Information:
Assertion failed: (backingScaleFactor > 0.f), function drawnFocusRingBitmapForTime, file /AppleInternal/BuildRoot/Library/Caches/, line 306.

I display the Color Picker with a Call :
Call Color.SelectedFromDialog(col, “”)

Some ideas of what can produce that?