Error with active window capture

[BobbyWibowo]

With -i $(xprop -root _NET_ACTIVE_WINDOW | grep -o '0x.*'), I'd keep on getting these errors with maim:

X Error of failed request:  BadWindow (invalid Window parameter)
  Major opcode of failed request:  3 (X_GetWindowAttributes)
  Resource id in failed request:  0x0
  Serial number of failed request:  21
  Current serial number in output stream:  22

I had to use -i $(xdotool getactivewindow) for it to work (thus also changing requirements from xprop to xdotool). maim used xdotool getactivewindow in their Git repo for active window capture, so I took it from that and it worked.

I'm on KDE neon 5.12.4 (Ubuntu 16.04 LTS), btw. Any idea what's up?

edit: I did NOT include this change in #37.