Make the 'Weight' field required in WooCommerce Edit Product page.

weight-required-field.php

<?php
/*
Plugin Name: Require Weight field (WooCommerce)
Plugin URI: https://charrua.es
Description: Make weight field required when adding/editing product. Usefull when using weight shipping.
Author: Daniel Pereyra Costas
Author URI: https://charrua.es
Version: 0.1
*/

add_action( 'admin_head', 'dcwd_require_weight_field' );
function dcwd_require_weight_field() {
	$screen         = get_current_screen();
	$screen_id      = $screen ? $screen->id : ''; 
	if ( $screen_id == 'product' ) {
	?>
		<script>
		jQuery(document).ready(function($){
			$('#_weight').prop('required',true);  		// Set weight field as required.

			$( '#publish' ).on( 'click', function() {
				weight = $('#_weight').val();
				if ( weight == '' || weight == 0  ) {
					alert( 'Debes completar el peso del artículo en la pestaña de envío' );

					$( '.shipping_tab > a' ).click();  	// Click on 'Shipping' tab.
					$( '#_weight' ).focus();  			// Focus on Weight field.
					return false;
				}
			});
		});
		</script>
	<?php
	}
}