If your theme or plugin is inserting files in MailPoet pages and it is breaking some functions, you should apply this conditional to load your files

MailPoet: Conditional to stop loading files in MailPoet pages

<?php
/**
 * If your theme or plugin is inserting files in MailPoet pages
 * and it is breaking some functions, you should apply this 
 * conditional to load your files.
 * 
 * Normally themes use these functions wp_enqueue_style() and wp_enqueue_script()
 */
if ( ( isset($_GET['page']) && $_GET['page'] != 'wysija_campaigns' ) && ( isset($_GET['page']) && $_GET['page'] != 'wysija_subscribers' ) && ( isset($_GET['page']) && $_GET['page'] != 'wysija_statistics' ) && ( isset($_GET['page']) && $_GET['page'] != 'wysija_config' ) ) {
	// Put your enqueue styles and scripts here 
}