Invalid argument supplied for foreach()

[jyhsu2000]

Environment

• PHP: 7.3.3 / 7.3.8
• Laravel: 5.8.33
• mews/purifier: 2.1.4
• ezyang/htmlpurifier: 4.10.0 (required by mews/purifier)
• config: default

Issue

When I use the following code to clean text with custom_definition config,

Purifier::clean($richText, 'custom_definition');

it throws Invalid argument supplied for foreach() at vendor/ezyang/htmlpurifier/library/HTMLPurifier/Config.php:691

Can you help for fix it? Or is there any workarounds to get over it?