Missing Menus Structure

Try adding add_theme_support( 'menus' ); in functions.php I haven’t taken the time to investigate. I suspect it depends on what theme is being resolved and how the functions.php is setup. also try to disable auto fill form browser.

did you register the menu?? if not then add this to your functions.php

function my_theme_setup() {
  register_nav_menus( array( 
    'header' => 'Header menu', 
    'footer' => 'Footer menu' 
  ) );
 }

add_action( 'after_setup_theme', 'my_theme_setup' );