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There are the following error messages instead of my website:

Warning: Declaration of Menu_With_Description::start_el(&$output, $item, $depth, $args) should be compatible with Walker_Nav_Menu::start_el(&$output, $item, $depth = 0, $args = Array, $id = 0) in /homepages/…./htdocs/mywebsitefolder/wp-content/themes/iam/functions.php on line 418

Warning: count(): Parameter must be an array or an object that implements Countable in /homepages/…./htdocs/mywebsitefolder/wp-content/themes/iam/7league/config.php on line 56

Warning: count(): Parameter must be an array or an object that implements Countable in /homepages/…./htdocs/mywebsitefolder/wp-content/themes/iam/7league/config.php on line 215

Warning: count(): Parameter must be an array or an object that implements Countable in /homepages/…./htdocs/mywebsitefolder/wp-content/themes/iam/7league/sidebars.php on line 66

How can I fix it?