WP Plugin Header Warning

I've been working on a simple plugin to modify the standard wp_editors. Everything works correctly, but I'm getting a strange error anytime I try to "save" my changes (Options - Settings API). I've read a lot of possible solutions to this issue, but I'm having a hard time finding a solution as I think is very specific to my plugin.

Here is a snippet of my code to establish some context (I removed some sections as its not necessary to show the full code)

<?php
/*
Plugin Name: mmm-editor
Plugin URI:  ---
Description: Customization for backend editors
Version:     1.0
Author:      Mateo Marquez
Author URI:  ---
License:     GPL2
License URI: http://ift.tt/XUg0iV
Domain Path: /languages
Text Domain: mmm
*/

//include & register fields
require_once('includes/mmm-settings.php');

//include fields functions
require_once('includes/mmm-fields.php');

//include tiny functions
require_once('includes/mmm-tiny.php');

//include ACF functions
require_once('includes/mmm-acf.php');


//add_action( 'admin_menu', 'mmm_add_admin_menu' );
add_action( 'admin_init', 'mmm_settings_init' );


function mmm_add_admin_menu(  ) {

    add_options_page( 'mmm-editor', 'WYGIWYS Options', 'manage_options', 'mmm-editor', 'mmm_options_page' );

}

function mmm_stylesheet( ) {
    wp_enqueue_style('admin-css', plugins_url('/assets/admin-styles.css', __FILE__));
    wp_enqueue_script('admin-js', plugins_url('/assets/admin-scripts.js', __FILE__), array('jquery'), '20120206', true );
}
add_action( 'admin_print_styles', 'mmm_stylesheet' );




function mmm_options_page(  ) {

    ?>
    <form action='options.php' method='post'>
        <div class="mmm-header">
            <h2>WYSIWYG Editor Options</h2>
        </div>

        <div class="sections">
            <?php
            settings_fields( 'globalEditor' );
            do_settings_sections( 'globalEditor' );
            ?>
        </div>


        <div class="sections">
            <?php
            settings_fields( 'topicsEditor' );
            do_settings_sections( 'topicsEditor' );
            ?>
        </div>

        <div class="sections">
            <?php
            settings_fields( 'forumsEditor' );
            do_settings_sections( 'forumsEditor' );
            ?>
        </div>

        <div class="sections">
            <?php
            settings_fields( 'blogsEditor' );
            do_settings_sections( 'blogsEditor' );
            ?>
        </div>

        <?php
        submit_button();
        ?>

    </form>
    <?php

}

Here is the error dialog that appears any time I try to "Save/submit" my form with the settings slected. Keep in mind this only happens when I got define('WP_DEBUG',true);. It doesn't seem to affect the behavior of my plugin ( I just have to reload or go back to access the settings page again -- but changes are saved). Even though this work, I don't want to use something with warnings in production. Here is the exact warning log:

Warning: Cannot modify header information - headers already sent by (output started at /Users/mmarquez/Sites/_______/wp-includes/functions.php:3792) in /Users/mmarquez/Sites/______/wp-includes/pluggable.php on line 1228

Any assistance is appreciated - I know this is a common question, but I've tried most of the other suggestions around.

Thanks!

via Chebli Mohamed