BUG: RichTextEditor is not oppening in IE11.

[Presentation2.pptx](https://github.com/artf/grapesjs/files/5438803/Presentation2.pptx)

        richTextEditor.add('insertHtml', {
            icon: '<select class="placeholders-select">' +
                    options()
                + '</select>',
                // Bind the 'result' on 'change' listener
            event: 'change',
            result: (rte, action) => rte.exec('insertHtml', action.btn.firstChild.value),
            update: (rte, action) => {
                const value = rte.doc.queryCommandValue(action.name)
                if (value != 'false') { // value is a string
                    action.btn.firstChild.value = value
                }
            },
        })