I’m come back to you because I found the solution. The ‘left zone’ option in Dash to Panel was desactived, so, the icon of the extension ‘show desktop button’ could not displayed to the left of the panel. After enabled it, it works.
I want to tell you that I tried another extension: Show Desktop Applet - GNOME Shell Extensions
because the ‘show desktop button’ extension (I talked above) remove the desktop icons when we use it. That’s not the case with ‘Show desktop applet’ extension.
I close this subject because I found the ‘workaround’. 