modal close : Uncaught TypeError: Cannot read property 'length' of undefined at V (cash.js:702)

Hi,

I've this code to display a modal when I click on my custom button.

const commands = editor.Commands;
commands.add('salva-template-command', {
    run: function (editor, sender, options) {

        testo = editor.runCommand('gjs-get-inlined-html');

        var modalTitle = 'My Title';
        var modalContent = '<h1> my content</h1>';

        const modal = editor.Modal;

        modal.setTitle(modalTitle);
        modal.setContent(modalContent);
        modal.open()
        modal.onceClose(() => this.stopCommand());

    },
    stop: function (editor) {
        editor.Modal.close();
    }
})

panelManager.addButton('commands', {
    id: 'btnSaveAsTemplate',
    className: 'fa fa-th-large',
    command: 'salva-template-command',
    attributes: { title: 'My titile' },
});  

At first click, the modal appears correctly. Then I close it. Now, if I click it again I've this error in console:

I'm able to show it again only if I click for another time the custom button.

I've already followed the instruction provided here --> https://grapesjs.com/docs/modules/Commands.html#stateful-commands but I'm not able to figure it out.

My GrapeJS version is : "0.14.62" (latest). I'm facing the issue with my local copy. Tested on Firefox (latest) and Chorme (latest) on Ubuntu 16.04 LTS.

Thanks in advance!