[SOLVED]: Drupal: Notice: Undefined variable: hide_site_name in include()

The error message below may be shown after changing the theme in Drupal 7.

• Notice: Undefined variable: hide_site_name in include() (line 98 of /home/XXXX/public_html/XXXX/sites/all/themes/XXXX/templates/page.tpl.php).
• Notice: Undefined variable: hide_site_name in include() (line 108 of/home/XXXX/public_html/XXXX/sites/all/themes/XXXX/templates/page.tpl.php).

The problem is due to the cache in Drupal, to resolve:

Step 1. Go to Configuration

Step 2. Go to Development -> Performance

Step 3. Clear Cache -> Clear all caches

Step 4. The error message is gone!