Error When Editing Page

Sorry, our developer who wrote this site is no longer available, and I’m not clever enough to work this out myself.

We have version 2.3.0 installed, which I believe is the latest version.

No custom code in the function.php, but we have this document in the themes/<themename>/widget_shared_gallery.php that has very similar code to that in the docs you provided.

<?php
	/**
	*	Create Widget
	*/
	class Shared_Gallery extends WP_Widget {
		// Class constructor
		function __construct() {
			parent::__construct(
				'shared-gallery', 
				'Shared Gallery',
				array( 
					'classname' => 'shared-gallery',
					'description' => 'Displays a gallery from an alternate blog on this network'
				)
			);
		}

		// Output for widget content
		public function widget($args, $instance) {

			echo $args['before_widget'];

			// if title is not null or empty
			if ($instance['title'] != null && $instance['title'] != '') 
			{
				// display the title
				echo '<h3 class="widget-title">'.$instance['title'].'</h3>';
			}

			$blogID = $instance['blogID'];
			$galleryID = $instance['galleryID'];

			// switch blog
			switch_to_blog($blogID);
			// display gallery
			nggShowGallery($galleryID, 'Terraplas', 9999);
			// restore blog
			restore_current_blog();

			// $query = 'SELECT filename, description, alttext FROM wp_'.wpdb::get_blog_prefix($blogID).'ngg_pictures WHERE galleryid = '.$galleryID;

			// global $wpdb;
			// $images = $wpdb->get_results( $query );

			// $str =  '<div>';
			// foreach ($images as $image) {
			// 	$str .=  '<img src="'.$image->filename.'" alt="'.$image->alttext.'"/>';
			// }
			// $str .=  '</div>';

			// echo $str;

			echo $args['after_widget'];
		}

		// Output for admin widget creation
		public function form($instance) {

	    	wp_enqueue_script( 'shared-gallery', get_stylesheet_directory_uri() . '/scripts/shared-gallery.js' );

			$blog_list = MslsBlogCollection::instance()->get_current_blog();

			$str = '<input type="hidden" id="blogID" name="'. esc_attr( $this->get_field_name( 'blogID' ) ) .'"/>';
			$str .= '<input type="hidden" id="galleryID" name="'. esc_attr( $this->get_field_name( 'galleryID' ) ) .'"/>';

			$str .= 	'<label>Gallery Title</label><br />';
			$str .= 	'<input type="text" id="'.esc_attr( $this->get_field_name( 'title' ) ).'" name="'. esc_attr( $this->get_field_name( 'title' ) ) .'" /><br />';

			$str .= 	'<label>Select a Blog</label><br />';
			$str .= 	'<select id="shared-gallery-blog" onchange="setSharedBlog()">';
			$str .=			'<option value=""> - Please Select a Blog - </option>';
			foreach ($blog_list as $blog) {
				$str .=		'<option value="'.$blog->blog_id.'">'.$blog->blogname.'</option>';
			}
			$str .= 	'</select><br /><br />';

			$str .= '<label id="shared-gallery-title" style="display:none;">Select a Gallery</label>';
			foreach ($blog_list as $blog) {
				$str .= '<select class="shared-gallery-gallery" id="shared-gallery-galleries-'.$blog->blog_id.'" onchange="setSharedGallery()" style="display:none;">';
				$str .=		'<option value=""> - Please Select a Gallery - </option>';

				$query = 'SELECT gid, title FROM wp_' . wpdb::get_blog_prefix($blog->blog_id) . 'ngg_gallery';

				global $wpdb;
				$results = $wpdb->get_results( $query );

				foreach ($results as $gallery) {
					$str .= '<option value="'.$gallery->gid.'">'.$gallery->title.'</option>';
				}
				$str .= '</select>';
			}

			echo $str;
		}

		// Update function for widget
		public function update($new_instance, $old_instance) {
			$instance = array();

			$instance['title'] = $new_instance['title'];

			if (!empty($new_instance['blog_gallery'])) 
			{
				$vars = array_split($new_instance['blog_gallery'], ':');

				$instance['blogID'] = $vals[0];
				$instance['galleryID'] = $vals[1];
			} else {
				$instance['blogID'] = $old_instance['blogID'];
				$instance['galleryID'] = $old_instance['galleryID'];
			}

			return $instance;
		}
	}

	// Register widget
	add_action( 'widgets_init', function(){
		register_widget( 'Shared_Gallery' );
	});
?>

That’s worked perfectly. Thanks for your help!