Need help with multiple taxonomy query

Hello. I am stuck on trying to use GET variables to inform a query modifying function I’ve written to retrieve custom post type posts that have two terms associated with them, each one being from a different custom taxonomy. Being quite novice, I’ve based this function off of code I’m using for a great search function I found on this site, which works well.

Here is what my function looks like (placed in functions.php) :

function two_tax_query($query) {

	if(is_tax('destination_county')) {

                $query->set('post_type', 'destinations');

		$county_slug =  $_GET['destination_county'];
		$dest_type =  $_GET['dest_type'];

		// create arrays because that is required by terms arg
                $county_slugs = array($county_slug);
		$dest_types = array($dest_type);

		$tax_query_args = array(
			array(
						'taxonomy' => 'destination_county',
						'field' => 'slug',
						'terms' => $county_slugs,
						'operator' => 'AND'
					),
			array(
						'taxonomy' => 'destination_category',
						'field' => 'slug',
						'terms' => $dest_types,
						'operator' => 'AND'
					)
		);
		$query->set('tax_query', $tax_query_args);
	}
}
add_action('pre_get_posts', 'two_tax_query', 1001);

The above function is run when WP encounters a query for the “destionation_county” custom tax type. It is started with a link from a menu I created by listing the counties terms and building the link in such a way as to include an extra GET variable. This extra GET variable is the term from the other tax type I would like to include into the query. Here’s what the links look like:

<a href="https://www.thewebsite.org/?destination_county=adams-county&dest_type=cultural" title="Adams County">Adams County</a>

Sadly, this doesn’t want to work. By that I mean no errors are thrown, but my site breaks in such a way that it seems every call to wp_nav_menu does not run.

Please let me know if this is going about it the wrong way. I would prefer not to use a plugin.