Merge pull request #72 from CodeWithDennis/revert-71-bugfix/nullable-type

Revert "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;