[FIX] web: action manager and modals should interact correctly

Before this commit, the action manager could in some case have a
reference of a modal dialog, even after it has been destroyed (for
example, when the "X" button has been clicked to close a modal).

Unfortunately, it has some unexpected consequences, for example, the
do_push_state method check the existence of self.dialog to update the
url.

addons/web/static/src/js/action_manager.js

@@ -691,6 +691,9 @@ var ActionManager = Widget.extend({
                     // underlying form view (see comments above)
                     pre_dialog.on_close();
                 }
+                if (!pre_dialog) {
+                    self.dialog = null;
+                }
             };
             this.dialog.on("closed", null, this.dialog.on_close);
             this.dialog_widget = executor.widget();