Problem with timezone after trying to set timezone

HI,i recently installed latest version of freepbx and does not have skill on it.when i try to set the time zone from GUI nothing happen then i try it from command and it said its OK.then i rebooted the system when it start again i saw this massage and i cant access to GUI because of black window in middle of page .
Whoops\Exception\ErrorException: date_default_timezone_set(): Timezone ID ‘/’ is invalid in file /var/www/html/admin/libraries/BMO/View.class.php on line 153
Stack trace:

  1. Whoops\Exception\ErrorException->() /var/www/html/admin/libraries/BMO/View. class.php:153
  2. date_default_timezone_set() /var/www/html/admin/libraries/BMO/View.class.ph p:153
  3. FreePBX\View->setTimezone() /var/www/html/admin/bootstrap.php:162
  4. require_once() /etc/freepbx.conf:9
  5. include_once() /var/lib/asterisk/bin/fwconsole:12
    Whoops\Exception\ErrorException: date_default_timezone_set(): Timezone ID ‘/’ is invalid in file /var/www/html/admin/libraries/BMO/View.class.php on line 153
    Stack trace:
  6. Whoops\Exception\ErrorException->() /var/www/html/admin/libraries/BMO/View. class.php:153
  7. date_default_timezone_set() /var/www/html/admin/libraries/BMO/View.class.ph p:153
  8. FreePBX\View->setTimezone() /var/www/html/admin/bootstrap.php:162
  9. require_once() /etc/freepbx.conf:9
  10. include_once() /var/lib/asterisk/bin/fwconsole:12