WP_DEBUG notice in some admin pages

I’m developing a site locally and I see an error on some admin pages when WP_DEBUG is on:

Notice: Trying to get property of non-object in /Users/dev/sites/local.dev/wp-content/plugins/am-events/am-events.php on line 292

I made a quick fix to the plugin on my installation to prevent this notice appearing by changing the first few lines in the am_custom_script_post function:

original:

	global $post;
	if ($post->post_type !== 'am_event' || !is_admin()) {
		return;
	}

changed to:

	global $pagenow;
	global $post;

	if ( !is_admin() || $pagenow !== 'post.php' || $post->post_type !== 'am_event' ) {
		return;
	}