Undefined offset: -1 error when displaying the_content(); in Genesis child theme

Question

I use this code to show the content from a post from a custom post type after the header on each page.

$args = [
    'numberposts'   => 1,
    'post_type'     => 'theme_elements',
    'meta_key'      => 'location',
    'meta_value'    => 'mega-menu'
];
    
$mega_menu_content = new WP_Query( $args );
    
if ( $mega_menu_content->have_posts() ): 
    
    while ( $mega_menu_content->have_posts() ) : $mega_menu_content->the_post();
    
            add_action( 'genesis_after_header', 'dex_add_mega_menu');
    
            function dex_add_mega_menu() { ?>
               <div class="dex-mega-menu dex-hidden">
                    <div class="dex-wrap">
                        <?php the_content(); ?>
                    </div>
                </div>
            <?php }  
    
    endwhile;
    
    wp_reset_query();
        
endif;

It works on the home page, but on all other pages I get this error:

Notice: Undefined offset: -1 in /wp-includes/post-template.php on line 325

I use the Genesis Framework.

What am I doing wrong here?

Answer 1

Found the answer myself.

I was adding the function to the 'genesis_after_header' hook, which is a template file hook. Instead, the query and relative functions should live outside that hook. I used output buffering to get the_content() into a variable and put that in the hooked function instead. See code below for my final, functioning code.

$args = [
    'numberposts'   => 1,
    'post_type'     => 'theme_parts',
    'meta_query'    => [
        [
            'key'   => 'location',
            'value' => 'mega-menu'
        ]
    ]
];

$mega_menu_post_query = new WP_Query( $args );

if ( $mega_menu_post_query->have_posts() ): 

    while ( $mega_menu_post_query->have_posts() ) : $mega_menu_post_query->the_post();

        ob_start();
        the_content();
        $content = ob_get_clean(); 

    endwhile;

    add_action( 'genesis_after_header', 'dex_add_mega_menu');

    function dex_add_mega_menu() { 
        global $content;
        ?>
        <div class="dex-mega-menu dex-hidden">
            <div class="dex-wrap">
                <?php echo $content ?>
            </div>
        </div>
    <?php }
    
endif;

wp_reset_query();