Fatal error in Customizer: theme_mod_header_image filter receives array

Hi,

I ran into a fatal error when trying to add a header video via the WordPress Customizer. Setting individual images on pages works, but when I try to open the Customizer to add a Video the error occurs.

The theme uses the native WordPress Custom Header feature with video support:

add_theme_support( 'custom-header', array(
  'default-image' => '',
  'width' => 1920,
  'height' => 1080,
  'flex-height' => false,
  'video' => true,
) );

and outputs it via:

the_custom_header_markup();

When opening the Customizer / trying to set a header video, the following fatal error occurs:

Fatal error: Uncaught TypeError: RyanHellyer\UniqueHeaders\DisplayModule::taxonomyHeaderImageFilter(): Argument #1 ($url) must be of type string, array given, called in wp-includes/class-wp-hook.php on line 341 and defined in wp-content/plugins/unique-headers/src/DisplayModule.php:77

Stack trace excerpt: 
0 wp-includes/class-wp-hook.php(341): RyanHellyer\UniqueHeaders\DisplayModule->taxonomyHeaderImageFilter(Array) 
1 wp-includes/plugin.php(205): WP_Hook->apply_filters(Array, Array) 
2 wp-includes/theme.php(1095): apply_filters('theme_mod_heade…', Array) 
3 wp-includes/class-wp-customize-setting.php(638): get_theme_mod('header_image', Array) 
4 wp-includes/class-wp-customize-setting.php(258): WP_Customize_Setting->get_root_value(Array) 
5 wp-includes/class-wp-customize-setting.php(216): WP_Customize_Setting->aggregate_multidimensional() 
6 wp-includes/class-wp-customize-manager.php(5408): WP_Customize_Setting->__construct(Object(WP_Customize_Manager), 'header_image', Array)

It looks like the plugin callback attached to theme_mod_header_image is too strictly typed:

public function taxonomyHeaderImageFilter(string $url): string

However, WordPress seems to pass an array in this Customizer context when calling get_theme_mod('header_image', Array). Since the theme_mod_{$name} filter can receive mixed values, this causes a fatal TypeError before the Customizer can load.

The same may also apply to:

public function postHeaderImageFilter(string $url): string

A possible fix would be to remove the strict string type declaration and return non-string values unchanged, for example:

public function taxonomyHeaderImageFilter($url) {
  if (! is_string($url)) {
    return $url;
  }
  // existing logic...
}

Same for postHeaderImageFilter().

This started happening after the recent plugin update. The theme itself does not modify the value before the Unique Headers filter runs; the plugin filter runs earlier and crashes as soon as WordPress passes the array value.

Could you please check this?

Thanks!