Bootbox dialog.modal('hide') doesn't hide modal

I have a problem with bootbox.js modals. I wanted to use bootbox.dialog for pausing UI while Ajax is being executed and waiting for response. Everything works cool if I use dialog.modal('hide'); inside bootbox.alert with confirm (after clicking OK it hides dialog.modal), but I'd not like to confirm it every time. When I'm using dialog.modal('hide'); outside the bootbox.alert with confirm it doesn't hide dialog.modal... Where is the problem? Working code (hiding modal) is inside success and not working is inside error

var dialog = bootbox.dialog({
            message: '<p class="text-center mb-0"><i class="fa fa-spin fa-cog"></i> Please wait while we do something...</p>',
            closeButton: false
        });
        var checkboxId = "#" + $(this).attr('id');
        var checked = $(this).is(":checked");
        if (checked) {
            $.ajax({
                url: url,
                type: "POST",
                data: { estimatedCostId: @Model.EstimatedCostID },
                success: function (data) {
                    if (!data.Success) 
                    {
                        bootbox.alert(data.ErrorMessage, function () {
                            dialog.modal('hide');
                        });
                        $(checkboxId).prop('checked', !checked);
                    }
                },
                error: function (request, status, error) {
                    dialog.modal('hide');
                    bootbox.alert("Błąd serwera");
                    $(checkboxId).prop('checked', !checked);
                }
            });
        }

Most likely, this is a timing issue. If your AJAX call is "too quick", the success/fail callbacks are being called before the bootbox.dialog function has resolved. So, this:

dialog.modal('hide');

winds up being called on an undefined object. I was running into a similar issue on a recent project, and solved it by putting the AJAX call into the shown.bs.modal event, like so:

var checkboxId = "#" + $(this).attr('id');
var checked = $(this).is(":checked");

var dialog = bootbox.dialog({
    message: '<p class="text-center mb-0"><i class="fa fa-spin fa-cog"></i> Please wait while we do something...</p>',
    closeButton: false
})
 // this is the change
.on('shown.bs.modal', function(){
    if (checked) {
        $.ajax({
            url: url,
            type: "POST",
            data: { estimatedCostId: @Model.EstimatedCostID },
            success: function (data) {
                if (!data.Success) 
                {
                    bootbox.alert(data.ErrorMessage, function () {
                        dialog.modal('hide');
                    });
                    $(checkboxId).prop('checked', !checked);
                }
            },
            error: function (request, status, error) {
                dialog.modal('hide');
                bootbox.alert("Błąd serwera");
                $(checkboxId).prop('checked', !checked);
            }
        });
    }
});