message: fix popup exception
diff: @@ -39,8 +39,10 @@ export class Popup extends SurveyElementBase<IPopupProps, any> {
  );
  }
  componentWillUnmount() {
  + if (this.container) {
  this.container.remove();
  this.container = undefined;
  + }
  super.componentWillUnmount();
  }
  }
  