dash: destroy items's child before tooltip

Destroy the DashItemContainer's child from the same handler as the tooltip. This
will prevent invalid reads when the item is destroyed while its quicklist is
still open.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/781

(cherry picked from commit ff2fbf5ae4)

js/ui/dash.js

@@ -52,6 +52,8 @@ var DashItemContainer = new Lang.Class({
         this.animatingOut = false;
 
         this.connect('destroy', () => {
+            if (this.child != null)
+                this.child.destroy();
             this.label.destroy();
         });
     },