grabHelper: Throw an error if the owner is not an actor

Starting from commit 7bb84dae, GrabHelper requires the owner to be an Actor as
we pass this to pushModal that assumes it to be as well.

So check that GrabHelper owner is an actor and throws an error if it is not the
case. This helps in tracking down issues such as gnome-shell-extensions!68

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/565

js/ui/grabHelper.js

@@ -43,6 +43,9 @@ function _popGrabHelper(grabHelper) {
 // call grab().
 var GrabHelper = class GrabHelper {
     constructor(owner, params) {
+        if (!(owner instanceof Clutter.Actor))
+            throw new Error('GrabHelper owner must be a Clutter.Actor');
+
         this._owner = owner;
         this._modalParams = params;