Hello again and thank you for answering.
I have tried to define WP_Category_Checkboxes_Control class in my functions.php file, wordpress first defines my class and the goes on to the parent theme and when it reaches to the same class in the parent theme, it gives an error because this class is already defined (in the child theme), and it breaks the website. The only way to fix this as I know, is to remove the class definition in the parent theme or add a line to check if the class is already defined and skip the definition if so.
I don’t know how else would I define the class before the parent theme.
