Code works perfectly standalone, but not with the rest of the html. I think it's because it's having trouble finding the child element to click on.
To test: Click on thumbnail photos on the bottom left.
document.querySelector('[data-target="1"]').addEventListener('click', function() {
var toolbar = document.getElementById('toolbar');
toolbar.style.visibility = "hidden";
}, false);
document.querySelector('[data-target="0"]').addEventListener('click', function() {
var toolbar = document.getElementById('toolbar');
toolbar.style.visibility = "visible";
}, false);
<div class="editor_thumbnail">
<ul id="side-switcher">
<li class="thumb product-switch"><img data-target="0" src="https://upload.wikimedia.org/wikipedia/commons/0/09/Icon_1_%28set_basic%29.png" alt="" class="tooltip img_switcher img d_block tooltipstered"></li>
<li class="thumb product-switch"><img data-target="1" src="https://upload.wikimedia.org/wikipedia/commons/thumb/3/3d/Icon_2_blue.svg/1024px-Icon_2_blue.svg.png"></li>
</ul>
</div>
<ul id="toolbar">
<li id="tab-ref-add_image" class="tab-link" data-tab="add_image"><a href="#" class="show"><i class="material-icons">add_a_photo</i><span class="icon-text txtLang">Add Art</span></a></li>
<li id="tab-ref-add_text" class="tab-link" data-tab="add_text"><a href="#" class="show"><i class="material-icons">font_download</i><span class="icon-text txtLang">Add Text</span></a></li>
<li id="tab-ref-add_layer" class="tab-link" data-tab="view_layer"><a href="#" class="show"><i class="material-icons">layers</i><span class="icon-text txtLang">Layers</span></a></li>
<li id="tab-ref-save_data" class="tab-link" data-tab="save_data"><a href="#" class="show"><i class="material-icons">save</i><span class="icon-text txtLang">Save Product</span></a></li>
<li id="tab-ref-load_saved" class="tab-link" data-tab="load_saved"><a href="#" class="show"><i class="material-icons">assignment_turned_in</i><span class="icon-text txtLang">Load</span></a></li>
</ul>
The white icons in the dark grey toolbar should hide and not work when the brown label thumbnail is clicked.
The white icons in the dark grey toolbar should show and work when the white shirt thumbnail is clicked.