[FIX] web: do not call method on undefined controller

As `create_view` is asynchronous, it is possible that there is an
active view without any associated controller.

We thus test here if there is a controller before calling its method.

addons/web/static/src/js/chrome/view_manager.js

@@ -178,7 +178,9 @@ var ViewManager = Widget.extend(ControlPanelMixin, {
             }
             core.bus.on('clear_uncommitted_changes', self, function(chain_callbacks) {
                 chain_callbacks(function() {
-                    return self.active_view.controller.canBeDiscarded();
+                    if (self.active_view.controller) {
+                        return self.active_view.controller.canBeDiscarded();
+                    }
                 });
             });
         });