In SquirrelMail 'None' is special keyword and can't be used as mailbox name.
sqimap_mailbox_select() function does not select mailbox.
displayPageHeader() function does not display 'Current folder: None' in top menu bar.
In 1.4.9cvs right_main.php does not display messages in None mailbox.
In 1.5.2cvs right_main.php creates PHP notices, because sqm_api_mailbox_select() expects array with IMAP SELECT command data and sqimap_mailbox_select() returns NULL.
You already have FIXME tag in 1.5.2cvs functions/imap_mailbox.php.
PHP notices can be reproduced only on servers that have empty string as personal namespace prefix. dovecot 0.99, hmailserver, mercury32 and other simple IMAP servers without namespace support are affected. cyrus and courier are not affected. uw is affected only when uw preset is not used.
If displayPageHeader() is fixed, it will break backwards compatibility and all plugins that depend on $mailbox='None' must be updated. If empty string is used instead of 'None', updated plugins are backwards compatible with older SquirrelMail versions.