You can extend it, but the class definition and any code that uses it, can’t run outside of the admin area.
Wrap your class in this conditional:
if ( is_admin() ) {
...
}
If you are trying to use this on the frontend or other non-Admin areas, do not do that.
Generally, classes WordPress uses should not be extended, walker classes and WP_Widget are the notable extensions.
Also, it’s good practice to put classes in their own files.
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