Bug(DesignManager): $subtype doesn't exist if unspecified

Fix error occurring when subtype isn't specified at all in a design request.

app/Services/DesignUpdateManager.php

@@ -391,7 +391,7 @@ public function saveRequestFeatures($data, $request) {
             if (!$species) {
                 throw new \Exception('Invalid species selected.');
             }
-            if ($subtype && $subtype->species_id != $species->id) {
+            if (isset($subtype) && $subtype && $subtype->species_id != $species->id) {
                 throw new \Exception('Subtype does not match the species.');
             }