Component/select: Before uppercasing $child->group, make sure it isn'…

…t null

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

lib/Component.php

@@ -249,7 +249,7 @@ function ($child) use ($group) {
         $result = [];
         foreach ($this->children as $childGroup) {
             foreach ($childGroup as $child) {
-                if ($child instanceof Property && strtoupper($child->group) === $group) {
+                if ($child instanceof Property && $child->group && strtoupper($child->group) === $group) {
                     $result[] = $child;
                 }
             }