old_contents: (function poll() {
    if (typeof ynabToolKit !== 'undefined' && ynabToolKit.pageReady === true && ynabToolKit.onCurrentRouteChangedInit === true) {
      ynabToolKit.stealingFromNextMonth = (function () {
        function onNextMonthCalculationChanged() {
          let availableToBudget = this.getAvailableToBudget();
  
          let value = $('.budget-header-totals-details-values .budget-header-totals-cell-value').eq(3);
          let name = $('.budget-header-totals-details-names .budget-header-totals-cell-name').eq(3);
  
          if (availableToBudget < 0) {
            value.addClass('ynabtk-stealing-from-next-month');
            name.addClass('ynabtk-stealing-from-next-month');
  
            $('#ynabtk-stealing-amount', name).remove();
            name.append('<span id="ynabtk-stealing-amount"> (<strong>' + ynab.formatCurrency(availableToBudget) + '</strong>)</span>');
          } else {
            value.removeClass('ynabtk-stealing-from-next-month');
            name.removeClass('ynabtk-stealing-from-next-month');
            $('#ynabtk-stealing-amount', name).remove();
          }
        }
  
        return {
          invoke() {
            let budgetController = ynabToolKit.shared.containerLookup('controller:budget');
            let budgetViewModel = budgetController.get('budgetViewModel');
            let nextMonth = budgetViewModel.get('monthlyBudgetCalculationForNextMonth');
  
            nextMonth.addObserver('availableToBudget', onNextMonthCalculationChanged);
            onNextMonthCalculationChanged.call(nextMonth);
          },
  
          onRouteChanged(currentRoute) {
            if (currentRoute.indexOf('budget') !== -1) {
              ynabToolKit.stealingFromNextMonth.invoke();
            }
          }
        };
      }());
  
      let currentRoute = ynabToolKit.shared.getCurrentRoute();
      ynabToolKit.stealingFromNextMonth.onRouteChanged(currentRoute);
    } else {
      setTimeout(poll, 250);
    }
  }());
  
new_contents: (function poll() {
    if (typeof ynabToolKit !== 'undefined' && ynabToolKit.pageReady === true && ynabToolKit.onCurrentRouteChangedInit === true) {
      ynabToolKit.stealingFromNextMonth = (function () {
        function onNextMonthCalculationChanged() {
          let availableToBudget = this.getAvailableToBudget();
  
          let value = $('.budget-header-totals-details-values .budget-header-totals-cell-value').eq(3);
          let name = $('.budget-header-totals-details-names .budget-header-totals-cell-name').eq(3);
  
          if (availableToBudget < 0) {
            value.addClass('ynabtk-stealing-from-next-month');
            name.addClass('ynabtk-stealing-from-next-month');
  
            $('#ynabtk-stealing-amount', name).remove();
            name.append('<span id="ynabtk-stealing-amount"> (<strong>' + ynab.formatCurrency(availableToBudget) + '</strong>)</span>');
          } else {
            value.removeClass('ynabtk-stealing-from-next-month');
            name.removeClass('ynabtk-stealing-from-next-month');
            $('#ynabtk-stealing-amount', name).remove();
          }
        }
  
        return {
          invoke() {
            let budgetController = ynabToolKit.shared.containerLookup('controller:budget');
            let budgetViewModel = budgetController.get('budgetViewModel');
            if (budgetViewModel) {
              let nextMonth = budgetViewModel.get('monthlyBudgetCalculationForNextMonth');
  
              nextMonth.addObserver('availableToBudget', onNextMonthCalculationChanged);
              onNextMonthCalculationChanged.call(nextMonth);
            }
          },
  
          onRouteChanged(currentRoute) {
            if (currentRoute.indexOf('budget') !== -1) {
              ynabToolKit.stealingFromNextMonth.invoke();
            }
          }
        };
      }());
  
      let currentRoute = ynabToolKit.shared.getCurrentRoute();
      ynabToolKit.stealingFromNextMonth.onRouteChanged(currentRoute);
    } else {
      setTimeout(poll, 250);
    }
  }());
  
subject: add a check for null return value from budgetContrller() to fix null reference exception.