Bootstrap 3 Modal and Knockout using ko.bindingHandlers['with']

Question

I am trying to incorporate a Bootstrap 3 modal in to a knockoutjs viewmodel with a custom binding, and have taken heavily from knockout.js - deferred databinding for modal?.

Using https://jsfiddle.net/BitWiseGuy/4u5egybp/ each observable on the popup is checked if it is valid i.e. data-bind="value: UserBeingEdited() && UserBeingEdited().Age and this will result in a lot of checks (to write) for a larger view-model with many fields.

So to remove this check i have tried to use modify the modal binding with return ko.bindingHandlers['with'].update.apply(this, arguments); as below:

ko.bindingHandlers['modal'] = {

    init: function(element, valueAccessor, allBindingsAccessor) {
        var allBindings = allBindingsAccessor();
        var $element = $(element);
        $element.addClass('hide modal');

        if (allBindings.modalOptions && allBindings.modalOptions.beforeClose) {
            $element.on('hide', function() {
                    var value = ko.utils.unwrapObservable(valueAccessor());
                    return allBindings.modalOptions.beforeClose(value);
                });
        }

        return ko.bindingHandlers['with'].init.apply(this, arguments);
    },

    update: function(element, valueAccessor) {
        var value = ko.utils.unwrapObservable(valueAccessor());

        if (value) {
            $(element).removeClass('hide').modal('show');
        } else {
            $(element).modal('hide');
        }

        return ko.bindingHandlers['with'].update.apply(this, arguments);
    }
};

Which then allows me to bind without any check data-bind="text: quantity", however although working i am getting an error: Uncaught TypeError: Cannot read property 'apply' of undefined.

I have the following questions please:

1. How can i remove the errors in the console with my fiddle http://jsfiddle.net/6eh3Lrsm/9/

2. As per the comments from the original SO link, the with binding recreates DOM elements which could stop controls from working - in which cases would I need to look out for this (and any links to why)? In these cases I presume not using with and doing a binding check within the data-bind value itself would be enough?