Solution:1
The problem is in your theme somewhere. If you have another theme installed (but obviously inactive), use this guide to change your theme to something else while you troubleshoot the problem.
Solution:2
Try opening theme-checker.php file and then wrap the class declaration code with the class_exists() function, something like:
if ( !class_exists( 'WP_Theme_Handler_Checker' ) {
//The existing class declaration
}