Return NULL if taking screenshot fails
Return NULL if taking screenshot fails

It can happen if the display is not yet ready.
This change just removes a couple of warnings.

dezelin/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,dezelin/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,dezelin/spice-gtk,guodong/spice-gtk,dezelin/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,mathslinux/spice-gtk,guodong/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,mathslinux/spice-gtk,elmarco/spice-gtk,mathslinux/spice-gtk,Fantu/spice-gtk,mathslinux/spice-gtk