Issue updating backbone pane in upload script

meta.php

<?php

add_action( 'save_post', 'myplugin_save_meta_box_data' );

add_action( 'admin_enqueue_scripts', function () {
	wp_enqueue_media();
} );

add_action( 'add_meta_boxes', 'myplugin_add_meta_box' );

add_action( 'admin_footer', function () {
	?>
	<style>.pdf-media-field img {
			max-width: 100%;
			height:    auto;
			padding:   3px;
		}

		.pdf-media-field a {
			display: inline-block;
		}</style>
	<script>

		(function ($, window, document, undefined) {

			var defaults = {
				addSelector   : '.pdf-add',
				removeSelector: '.pdf-remove',
				inputSelector : '.pdf-id',
				thumbSize     : 'medium',
				modalOptions  : {}
			};

			function metaPdf(element, options) {
				this.$element = $(element);
				this.settings = $.extend({}, defaults, options);
				this._defaults = defaults;
				this.init();
			}

			metaPdf.prototype = {

				init: function () {
					this.$addLink = this.$element.find(this.settings.addSelector);
					this.$removeLink = this.$element.find(this.settings.removeSelector);
					this.$inputField = this.$element.find(this.settings.inputSelector);
					this.listen();
				},

				listen: function () {
					var _this = this;
					this.$addLink.on('click', function (e) {
						e.preventDefault();
						_this.openModal();
					});
					this.$removeLink.on('click', function (e) {
						e.preventDefault();
						_this.removeImage();
						$(this).hide();
					});
				},

				openModal: function () {
					var defaultOptions = {
						title: 'Select Media'
					};
					var options = $.extend(true, {}, defaultOptions, this.settings.modalOptions);
					this.modal = wp.media.frames.file_frame = wp.media(options);
					this.modalListen();
					this.modal.open();
				},

				modalListen: function () {
					var _this = this;
					this.modal.on('toolbar:create:select', function () {
						_this.modal.state().set('filterable', 'uploaded');
					});
					this.modal.on('select', function () {
						_this.getModalAttachment();
					});
					this.modal.on('open activate', function () {
						_this.attachmentToModal();
					});
				},

				getModalAttachment: function () {
					var attachments = [];
					var selections = this.modal.state().get('selection').models;
					$.each(selections, function () {
						attachments.push(this.toJSON());
					});
					this.attachImage(attachments);
				},

				attachmentToModal: function () {
					var attachments = this.$inputField.val().split(',');
					if (attachments) {
						var Attachment = wp.media.model.Attachment;
						var selection = this.modal.state().get('selection');
						if (typeof attachments == 'object') {
							$.each(attachments, function () {
								selection.add(Attachment.get(this));
							});
						}
						else {
							selection.add(Attachment.get(attachments));
						}
					}
				},

				attachImage: function (attachments) {
					var ids = [];
					var urls = [];
					var _this = this;
					$.each(attachments, function (i, attachment) {
						var url = _this.getThumbUrl(attachment);
						ids.push(attachment.id);
						urls.push(url);
					});
					this.$inputField.val(ids);
					this.setThumbHTML(urls);
					this.hasImage = true;
					this.$removeLink.show();
				},

				getThumbUrl: function (attachment) {
					var img_url = "";
					if (typeof attachment.sizes != "object") {
						img_url = attachment.icon;
					}
					else if (typeof attachment.sizes[this.settings.thumbSize] != "undefined") {
						img_url = attachment.sizes[this.settings.thumbSize].url;
					}
					else if (typeof attachment.sizes.medium != "undefined") {
						img_url = attachment.sizes.medium.url;
					}
					else {
						img_url = attachment.sizes.full.url;
					}
					return img_url;
				},

				removeImage: function () {
					this.$inputField.val('');
					this.$addLink.html(this.$addLink.attr('title')).addClass('button');
					this.hasImage = false;
				},

				setThumbHTML: function (urls) {
					_this = this;
					this.$addLink.html('').removeClass('button');
					$.each(urls, function (i, url) {
						var $img = $('<img>');
						$img.attr('src', url);
						_this.$addLink.append($img);
					});
				}

			};

			$(document).ready(function () {
				$('.pdf-media-field').each(function (i, e) {
					new metaPdf(this, $(this).data('pdf-field-settings'));
				});
			});

		})(jQuery, window, document);
	</script>
<?php
} );

function myplugin_add_meta_box() {

	$screens = array( 'post', 'page' );

	foreach ( $screens as $screen ) {

		add_meta_box(
			'myplugin_sectionid',
			__( 'My Post Section Title', 'myplugin_textdomain' ),
			'myplugin_meta_box_callback',
			$screen
		);
	}
}



/**
 * Prints the box content.
 *
 * @param WP_Post $post The object for the current post/page.
 */
function myplugin_meta_box_callback( $post ) {

// Add an nonce field so we can check for it later.
	wp_nonce_field( 'myplugin_meta_box', 'myplugin_meta_box_nonce' );

	$mime_types      = array( 'application/pdf' );
	$thumb_size      = 'medium';
	$label_add       = 'Set pdf';
	$label_remove    = 'Remove pdf';
	$link_content    = 'Link';
	$value_string    = 'Link';
	$field_name      = $field_id = 'thepdf';
	$hide_remove     = true;
	$value           = get_post_meta( $post->ID, 'thepdf_meta_key' );

	if ( $value ) {
		if ( ! is_array( $value ) ) {
			$value = array( $value );
		}
		$value_string = implode( ',', $value );
		foreach ( $value as $attachment ) {
			$value_post = get_post( $attachment );
			if ( $value_post ) {
				$mime_type      = $value_post->post_mime_type;
				$icon           = ( strpos( $mime_type, 'image' ) ) ? false : true;
				$thumbnail_html = wp_get_attachment_image( $attachment, $thumb_size, $icon );
				if ( ! empty( $thumbnail_html ) ) {
					$link_content .= $thumbnail_html;
					$hide_remove = false;
				}
				$link_content .= '<br>' . get_the_title( $value_post->ID );
			}
		}
	}
	// If no thumbnails then use link text and hide remove
	if ( empty( $link_content ) ) {
		$link_content = esc_html( $label_add );
		$hide_remove  = true;
	}
	// Settings for the the js object
	$field_settings = array(
		'thumbSize'    => $thumb_size,
		'modalOptions' => array(
			'multiple' => false,
			'title'    => $label_add,
			'button'   => array(
				'text' => $label_add
			),
			'library'  => array(
				'type' => $mime_types
			)
		)
	);
	?>
	<div class="pdf-media-field hide-if-no-js" data-pdf-field-settings="<?php echo esc_attr( json_encode( $field_settings ) ); ?>">
		<p>select a file</p>

		<p>
			<input class="hidden pdf-id" type="hidden" id="<?php echo $field_id; ?>" name="<?php echo $field_name; ?>" value="<?php echo esc_attr( $value_string ); ?>" />
			<a title="<?php echo esc_attr( $label_add ); ?>" href="#" class="pdf-add <?php echo ( $hide_remove ) ? 'button' : ''; ?>">
				<?php echo $link_content; ?>
			</a>
		</p>

		<p>
			<a href="#" class="pdf-remove button" <?php echo ( $hide_remove ) ? 'style="display:none;"' : ''; ?>>
				<?php echo esc_html( $label_remove ); ?>
			</a>
		</p>
	</div>
<?php
}

/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
function myplugin_save_meta_box_data( $post_id ) {

	/*
	* We need to verify this came from our screen and with proper authorization,
	* because the save_post action can be triggered at other times.
	*/

// Check if our nonce is set.
	if ( ! isset( $_POST['myplugin_meta_box_nonce'] ) ) {
		return;
	}

// Verify that the nonce is valid.
	if ( ! wp_verify_nonce( $_POST['myplugin_meta_box_nonce'], 'myplugin_meta_box' ) ) {
		return;
	}

// If this is an autosave, our form has not been submitted, so we don't want to do anything.
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}

// Check the user's permissions.
	if ( isset( $_POST['post_type'] ) && 'page' == $_POST['post_type'] ) {

		if ( ! current_user_can( 'edit_page', $post_id ) ) {
			return;
		}

	} else {

		if ( ! current_user_can( 'edit_post', $post_id ) ) {
			return;
		}
	}

	/* OK, it's safe for us to save the data now. */

// Make sure that it is set.
	if ( ! isset( $_POST['thepdf'] ) ) {
		return;
	}

// Sanitize user input.
	$my_data = sanitize_text_field( $_POST['thepdf'] );

// Update the meta field in the database.
	update_post_meta( $post_id, 'thepdf_meta_key', $my_data );
}