I used the answer on Have application menu open up with only Windows key to open the applications dock with Win key (works perfectly). Is there a way to have it close again using Win key? It just stays there unless I manually click somewhere else on the screen.
Please, use my updated answer:
gsettings set org.gnome.mutter overlay-key "'Super_L'"
gsettings set org.pantheon.desktop.gala.behavior overlay-action "'wingpanel --toggle-indicator=app-launcher'"
This solution works fine both on open and close Slingshot (the application launcher).