Activating sidebar widgets causes xhtml to not validate

I checked the validation before applying sidebar widgets, and it validated with no errors or warnings. When I add any sidebar widgets (the regular archive, categories, etc.), the site not longer validates. I get this error:

document type does not allow element "li" here; missing one of "ul", "ol", "menu", "dir" start-tag

I have searched and found that this means I have an li tag not within a ul tag, but I have checked and found none that seem to be miscoded. Here is the section of source code it says is causing the error:

#
# <div id="sidebar1">
#
# <li id="archives" class="widget widget_archives"><h2 class="widgettitle">Archives</h2>
# <select name="archive-dropdown" onchange='document.location.href=this.options[this.selectedIndex].value;'> <option value="">Select Month</option> <option value='https://focusorganic.com/wp/2008/01/'> January 2008 </option>
# <option value='https://focusorganic.com/wp/2007/12/'> December 2007 </option>
# </select>
# </li>
# </div> <!-- [sidebar1] -->

Specifically, line 502.

As it is now, I’ve only put one widget in the sidebar while I am trying to fix this problem. A page showing this problem can be seen here: https://focusorganic.com/wp/testest/