Merge pull request #71 from CodeWithDennis/bugfix/nullable-type

Make `customKey()` nullable

Author: CodeWithDennis

src/SelectTree.php

@@ -37,7 +37,7 @@ class SelectTree extends Field implements HasAffixActions
 
     protected bool $independent = true;
 
-    protected string $customKey = null;
+    protected ?string $customKey = null;
 
     protected string $titleAttribute;