Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, non-static method Widget_Tree_Nav::register() cannot be called statically in /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/class-wp-hook.php:287 Stack trace: #0 /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array) #1 /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/plugin.php(478): WP_Hook->do_action(Array) #2 /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/widgets.php(1773): do_action('widgets_init') #3 /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/class-wp-hook.php(287): wp_widgets_init('') #4 /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array) #5 /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/plugin.php(478): WP_Hook->do_action(Array) #6 /customers/2/9/b/skumfod.dk/httpd.www/wp-settings.php(546): do_action('init') #7 /customers/2/9/b/skumfod.dk/httpd.www/wp-config.php(89): require_once('/customers/2/9/...') #8 /customers/2/9/b/skumfod.dk/httpd.www/wp-load.php(37): require_once('/customers/2/9/...') #9 /customers/2/9/b/skumfod.dk/httpd.www/wp-blog-header.php(13): require_once('/customers/2/9/...') #10 /customers/2/9/b/skumfod.dk/httpd.www/index.php(17): require('/customers/2/9/...') #11 {main} thrown in /customers/2/9/b/skumfod.dk/httpd.www/wp-includes/class-wp-hook.php on line 287