JS errors when using a different burger

Problem/Motivation

Our custom theme is based on OpenCulturas Base. However, we chose to implement the menu toggle (burger) with different markup, by overriding page.html.twig.

Now we run into this JS error:

Uncaught TypeError: burgerButtonOpen is null
    menuDialogCleanup [host]/profiles/contrib/openculturas-distribution/profile/themes/openculturas_base/js/openculturas-base.js?s7emi6:198

Proposed resolution

The error is caused by the fact that function menuDialogCleanup tries to update elements that might not exist, in our case the open (#button-offcanvas-open) and close (#button-offcanvas-close) buttons.

It should return early when any of the necessary variables does not exist.