get_active_window gives back None sometimes

[kgabor545]

especially with mouse wheel set to prev/next window on group.
This caused group has active window false, making the group active, inactive and active again, caused flickering
Better have prev window as active than none, so the flickering is gone.
As I tested everything works well with this patch, I hope I didn't break anything else with this.

[xuzhen]

What's your desktop environment or window manager? I can't reproduce the flickering bug.

[kgabor545]

I am on arch xfce.
Have you tried to set the mouse wheel switch between windows in the group? (this how unity worked)
About once in every 10 switch, the button marker switch to inactive mode for a very short period of time, than switches back to active. Maybe if your marker is small and your machine is fast, you can't notice. If so, maybe try a theme with bigger marker.

I am currently using the xfce4 panel version of dockx, but the flickering was always here for me with native dockx too.
I have no issue since I added this modification.