Steps to reproduce the issue

In Joomla 3.8.13, go and create a New Menu Item. (The menu type doesn't matter, but we will be using a Category List to reproduce this issue). From the menu item type, select Articles -> Category List. From the 'Choose a Category' option, click 'Select' - if you now inspect the HTML, you will notice that there are two divs created:

<div class="modal-backdrop fade in"></div>
<div class="modal-backdrop fade in"></div>

When selecting the required 'Category' for the Category List, the CMS closed one of the above divs but the other is left open.

I have tested this in Chrome 69, Chrome 70, Firefox 62 and the above bug is always present.

If you don't select a Category, but click the X, this removes both of the divs.

Expected result

There shouldn't be two divs created when clicking on the Select Category

Actual result

Two divs created:

<div class="modal-backdrop fade in"></div>
<div class="modal-backdrop fade in"></div>

System information (as much as possible)

Additional comments