I’m seeing the same issue – there is a jQuery error – for some reason the SQL statement is getting evaluated in jQuery. FWIW I checked and the query itself is okay as an SQL query.
jquery.min.js?ver=3.5.1:formatted:566 Uncaught Error: Syntax error, unrecognized expression: SELECT tr_id, tr_parent_id, tr_title, tr_link, tr_active_link, tr_hide, tr_thumbnail_url, tr_type, (select count(trp.tr_id) from wp_bac_wm_trees trp where trp.tr_parent_id = tr.tr_id and tr_title <> ”) as c
FROM wp_bac_wm_trees tr
WHERE tr_title <> ” and mp_id = 1 AND tr_parent_id IS NULL ORDER BY tr_title ASC
<li id=”1″ class=”wm-li-node jstree-closed”
data-jstree='{“icon”:””}’> <span>stag.beechaeroclub.org</span> <span style=”color: #AFA03F;” onclick=”wm_editVerNode(1)”><i class=”fa fa-pencil-square-o”></i></span> <span style=”color: #41A34B;” onclick=”wm_add_ver_node(1)”><i class=”fa fa-plus-square”></i></span> <span class=”” onclick=”wm_hide_ver_node(1,this)”><i class=”fa fa-eye-slash”></i></span>
at Function.se.error (jquery.min.js?ver=3.5.1:formatted:566)
at se.tokenize (jquery.min.js?ver=3.5.1:formatted:989)
at se.select (jquery.min.js?ver=3.5.1:formatted:1044)
at Function.se (jquery.min.js?ver=3.5.1:formatted:299)
at Function.s.find (jquery-migrate.min.js?ver=3.3.2:2)
at S.fn.init.find (jquery.min.js?ver=3.5.1:formatted:1158)
at s.fn.init.S.fn.init (jquery.min.js?ver=3.5.1:formatted:1179)
at new s.fn.init (jquery-migrate.min.js?ver=3.3.2:2)
at S (jquery.min.js?ver=3.5.1:formatted:55)
at e.jstree.plugins.checkbox.<anonymous> (jstree.min.js?ver=5.6:2)