appDisplay: Call super.vfunc_unmap last

So as to guarantee the unmapped state sticks and doesn't get toggled
back to mapped before we return.

Being in a mapped state when `FolderIcon.vfunc_unmap()` returned was
causing an assertion failure in `clutter_actor_set_mapped` and crashed
the shell.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/2170

js/ui/appDisplay.js

@@ -1525,10 +1525,10 @@ var FolderIcon = GObject.registerClass({
     }
 
     vfunc_unmap() {
-        super.vfunc_unmap();
-
         if (this._dialog)
             this._dialog.popdown();
+
+        super.vfunc_unmap();
     }
 
     open() {