add null return in ModalBase and remove mountModal() calls

...luebutton-html5/imports/ui/components/about/component.jsx

...5/imports/ui/components/logout-confirmation/component.jsx

...tton-html5/imports/ui/components/modal/base/component.jsx

...mponents/presentation/presentation-uploader/component.jsx

...button-html5/imports/ui/components/settings/component.jsx