fix: avoid warning for network event (#3942)

Author: leogermani

includes/data-events/connectors/class-esp-connector.php

@@ -41,7 +41,10 @@ public static function register_handlers() {
 		Data_Events::register_handler( [ __CLASS__, 'subscription_renewal_attempt' ], 'subscription_renewal_attempt' );
 		Data_Events::register_handler( [ __CLASS__, 'newsletter_updated' ], 'newsletter_subscribed' );
 		Data_Events::register_handler( [ __CLASS__, 'newsletter_updated' ], 'newsletter_updated' );
-		Data_Events::register_handler( [ __CLASS__, 'network_new_reader' ], 'network_new_reader' );
+
+		if ( class_exists( 'Newspack_Network\Initializer' ) ) {
+			Data_Events::register_handler( [ __CLASS__, 'network_new_reader' ], 'network_new_reader' );
+		}
 	}
 
 	/**