Bugfix: compat php72

app/admin/racks/edit-rack-devices.php

@@ -188,7 +188,7 @@ $(document).ready(function(){
                         // back side
                         foreach ($rack_contents as $d) {
                             for($m=$d->rack_start; $m<=($d->rack_start+($d->rack_size-1)); $m++) {
-                                if(array_key_exists($m, $available_back)) {
+                                if(is_array($available_back) && array_key_exists($m, $available_back)) {
                                     unset($available_back[$m]);
                                 }
                             }