Checks for relatedTarget

If related target is undefined then we can't complete the below code

app/assets/javascripts/gl_dropdown.js.coffee

@@ -217,6 +217,7 @@ class GitLabDropdown
       if e.which is 27 # Escape key
         $('.dropdown-menu-close', @dropdown).trigger 'click'
     @dropdown.on 'blur', 'a', (e) =>
+      if e.relatedTarget?
         $relatedTarget = $(e.relatedTarget)
         $dropdownMenu = $relatedTarget.closest('.dropdown-menu')