Unable to "add a child page" to a Group Handbook

Drupal raise this error:
"warning: Invalid argument supplied for foreach() in /Users/detro/Portals/local.drupal/includes/form.inc on line 637."

Looking into the code, the error is caused from the "$choices" variable is NULL when used in the "foreach" statement: this is in the body of the function "form_select_options", used here to generate the "options-tag" for the "select" (the drop-down-list) of the "Parent Page".