this.$toggleWeightBUtton is undefined

Problem/Motivation

Displaying a tabledrag, the following error is displayed in the console:

Uncaught TypeError: this.$toggleWeightButton is undefined

Since 23 July 2020, a new way to display tabledrag has been commited on core #3084916: Add a new Drupal.theme JavaScript function for theming the the show/hide row weight toggle and its wrapper in draggable tables, explained in changelog #3160782: Tabledrag's show/hide weights button is now rendered by theme functions.

Bootstrap extends Drupal.tableDrag and call its parent initColumns() method. This method will call itself the displayColumns method which expect $toggleWeightButton to be a jQuery object.

// In Drupal core/misc/tabledrag.js.
  Drupal.tableDrag.prototype.displayColumns = function (displayWeight) {
    if (displayWeight) {
      this.showColumns();
    } else {
        this.hideColumns();
      }

    this.$toggleWeightButton.html(Drupal.theme('toggleButtonContent', displayWeight));
    $('table').findOnce('tabledrag').trigger('columnschange', !!displayWeight);
  };

Otherwise, during its override, Bootstrap does not initiate this property.

// In bootstrap js/misc/tabledrag.js.
    // Initialize the specified columns (for example, weight or parent columns)
    // to show or hide according to user preference. This aids accessibility
    // so that, e.g., screen reader users can choose to enter weight values and
    // manipulate form elements directly, rather than using drag-and-drop..
    self.initColumns();

Steps to reproduce

Open the console in your browser development tools to display javascript errors.
Display a form with drag&drop field in bootstrap theme.

Proposed resolution

Initiate and use the Object.$toggleWeightButton instead of define its own $button variable.