windowManager: Make sure to reset the resizePopup after using it

So that we'll recreate it the next time we want to show it. Otherwise,
we'll try to call things on a half-destroyed ResizePopup and end up
causing errors instead of showing the user their resize popup.

js/ui/windowManager.js

@@ -1561,8 +1561,10 @@ const WindowManager = new Lang.Class({
 
             this._resizePopup.set(rect, displayW, displayH);
         } else {
-            if (this._resizePopup)
+            if (this._resizePopup) {
                 this._resizePopup.destroy();
+                this._resizePopup = null;
+            }
         }
     },
 });