Thanks for pointing me in the correct direction for find the error logs. from there, I found this error message
PHP Fatal error: Uncaught Error: Call to a member function addJavascript() on null in /core/components/tinymce/tinymce.class.php:151
and from there a quick Google search led me to Call to a member function addJavascript() on a non-object · Issue #50 · BobRay/newspublisher · GitHub where I solved the issue by commenting out $this->modx->invokeEvent('OnRichTextEditorInit', $fields);