Thank @james , I can save thumbnail with your link.
The "Error message" popped up when I click on Exit too quickly after clicking on Save button !
I have always the same problem when I click on Save button and then Save Settings button , the thumbnail becomes all black .
This is the rapport with firebug
thumbnail.txt (25.7 KB)
I 've opened the console before clicking on Save button .
If you read this file with notepad for example , the action "click on Save" is after line 74 , the action " click on Save Settings" is after line 121 and the action " click on Exit " is after line 154.
It is difficult for me to see a problem