Widgets & Navigation Menus

<?php
defined( 'ABSPATH' ) || exit;

// has_nav_menu( string $location ): bool - true only when a menu is assigned
// to the location in Appearance > Menus.
if ( has_nav_menu( 'acme-events-primary' ) ) {
	wp_nav_menu(
		array(
			'theme_location'  => 'acme-events-primary',
			'container'       => 'nav',            // 'div' by default; false for none
			'container_class' => 'acme-nav',
			'container_aria_label' => __( 'Events navigation', 'acme-events' ), // WP 5.5
			'menu_class'      => 'acme-nav__list',
			'depth'           => 2,                // 0 = unlimited
			// %1$s = menu_id, %2$s = menu_class, %3$s = the <li> items.
			'items_wrap'      => '<ul id="%1$s" class="%2$s">%3$s</ul>',
			'fallback_cb'     => false,            // never fall back to wp_page_menu
		)
	);
}

// 'echo' => false returns the markup instead of printing it, which is what a
// shortcode or a REST callback needs. Returns false if there is no menu.
$markup = wp_nav_menu(
	array(
		'theme_location' => 'acme-events-primary',
		'echo'           => false,
		'fallback_cb'    => false,
	)
);

if ( is_string( $markup ) ) {
	return $markup;
}

<?php
defined( 'ABSPATH' ) || exit;

class Acme_Upcoming_Events_Widget extends WP_Widget {

	public function __construct() {
		parent::__construct(
			'acme_upcoming_events',                        // $id_base
			__( 'Upcoming Events', 'acme-events' ),        // $name
			array( 'description' => __( 'A list of the next events.', 'acme-events' ) )
		);
	}

	/** Front-end output. $args holds before_widget etc from register_sidebar(). */
	public function widget( $args, $instance ) {
		echo $args['before_widget'];

		if ( ! empty( $instance['title'] ) ) {
			echo $args['before_title'] . esc_html( $instance['title'] ) . $args['after_title'];
		}

		echo do_shortcode( '[acme_events limit="' . absint( $instance['limit'] ?? 3 ) . '"]' );
		echo $args['after_widget'];
	}

	/** The admin form. */
	public function form( $instance ) {
		printf(
			'<p><label for="%1$s">%2$s</label><input class="widefat" id="%1$s" name="%3$s" type="text" value="%4$s" /></p>',
			esc_attr( $this->get_field_id( 'title' ) ),
			esc_html__( 'Title:', 'acme-events' ),
			esc_attr( $this->get_field_name( 'title' ) ),
			esc_attr( $instance['title'] ?? '' )
		);
	}

	/** Sanitise before saving. Return false to cancel the save. */
	public function update( $new_instance, $old_instance ) {
		return array(
			'title' => sanitize_text_field( $new_instance['title'] ?? '' ),
			'limit' => absint( $new_instance['limit'] ?? 3 ),
		);
	}
}

add_action( 'widgets_init', static function (): void {
	register_widget( 'Acme_Upcoming_Events_Widget' );
} );

<?php
defined( 'ABSPATH' ) || exit;

// From a theme: remove the support core adds by default in WordPress 5.8+.
// Must run after core registers it, so after_setup_theme is the place.
add_action( 'after_setup_theme', static function (): void {
	remove_theme_support( 'widgets-block-editor' );
} );

// From a plugin or mu-plugin, where you should not speak for the theme:
// use_widgets_block_editor is a bool filter, so __return_false is the
// whole callback - it is core's shorthand for function () { return false; }.
add_filter( 'use_widgets_block_editor', '__return_false' );

// Equivalent longhand, if you need to make the decision conditional:
add_filter( 'use_widgets_block_editor', static function ( bool $use ): bool {
	// Keep blocks for administrators, classic widgets for everyone else.
	return current_user_can( 'manage_options' ) ? $use : false;
} );