There is a mistake in this code. The ToggleModal should always set NextModalState to ShowModal parameter. The first four lines should be:
function ToggleModal(ShowModal) { // ShowModal Boolean: True mean show
NextModalState = ShowModal;
if (! ModalIsRendering) {
The bad code will in most cases remove the modal upon drawing it, as the next mode was not set.
UPDATE WARNING:
For the above to work, you need to disable the close button and the click outside of modal features of the Modal. NOTE: in a later post a better method of removing the close button is shown.