PHP Wordpress checkbox issue

I am building a wordpress plugin with the boilerplate framework (https://wppb.me/). That all works find. I have an annoying issues which I cannot solve.

I have a table with a Boolean field AM_ACTIVE

In the page layout I want this to be an checkbox. See method below. A array is initialized and with the shortcode_atts the array and the request parameters are merged. For an add new this works. But if I edit it. The edit page is showing the checked/non checked correctly. But save will always result in false. I have tried a million things. With True/false 0/1. Putting the value with cache values, item Values, request values. Most of the time I get an PHP Warning: A non-numeric value encountered in /var/www/vhosts/marcelvreuls.eu/httpdocs/wp-content/plugins/am

So I assume the checkbox is not know when it is not touched or something. I have no clue. Can anybody help me out. I just want to activated/deactived a record 😉.

Any help will be highly appreciated

First the page code, after that the save code

    function amarre_maingroup_form_meta_box_handler($item)
    {
        ?>
        <table cellspacing="2" cellpadding="5" style="width: 100%;" class="form-table">
            <tbody>
            <tr class="form-field">
                <th valign="top" scope="row">
                    <label for="am_name"><?php _e('Name', 'amarre')?></label>
                </th>
                <td>
                    <input id="am_name" name="am_name" type="text" style="width: 50%" value="<?php echo esc_attr($item['am_name'])?>"
                           size="50" class="code" placeholder="<?php _e('am_maingroup', 'amarre')?>" required>
                </td>
            </tr>         
            <tr class="form-field">
                <th valign="top" scope="row"> 
                    <label for="am_url"><?php _e('Afbeelding', 'amarre')?></label>
                </th>
                <td>
                    <input id="am_url" name="am_url" type="text" style="width: 50%" value="<?php echo esc_attr($item['am_url'])?>"
                           size="50" class="code" placeholder="<?php _e('Afbeelding', 'amarre')?>" required>
                </td>
            </tr>        
             <tr class="form-field">
                <th valign="top" scope="row">
                    <label for="am_active"><?php _e('am_active', 'amarre')?></label><?php echo $_POST['am_active']?>
                </th>
                <td>
                <?php 
                if ($item['am_active'] == 1)
                {
                    echo('<input name="am_active" type="checkbox"  value="1" checked="checked">');
                }
                else
                {
                    echo('<input name="am_active" type="checkbox"  value="0">' );
                }              
                ?>
                </td>
            </tr>      
            </tbody>
        </table>
        <?php
    }

function amarre_maingroup_form_page_handler()
    {
        global $wpdb;
        $table_name = $wpdb->prefix . 'am_maingroup';

        $message = '';
        $notice = '';

        // this is default $item which will be used for new records
        $default = array(
            'id' => 0,
            'am_name' => '',
            'am_active' => '',
            'am_url' => ''
        );

        // here we are verifying does this request is post back and have correct nonce
        if ( isset($_REQUEST['nonce']) && wp_verify_nonce($_REQUEST['nonce'], basename(__FILE__))) {
            // combine our default item with request params
            $item = shortcode_atts($default, $_REQUEST);

            error_log( implode( ", ", $_REQUEST ));
            error_log( implode( ", ", $item ));
            // validate data, and if all ok save item to database
            // if id is zero insert otherwise update
            $item_valid =   $this->amarre_validate_maingroup($item);
            if ($item_valid === true) {  
                error_log( $item['am_name']);    
                error_log( $_REQUEST['am_active']);              
                if ($item['id'] == 0) {
                    $result = $wpdb->insert($table_name, $item);
                    $item['id'] = $wpdb->insert_id;
                    if ($result) {
                        $message = __('Item was successfully saved', 'amarre');
                    } else {
                        $notice = __('There was an error while saving item' + $result, 'amarre');
                    }
                } else {
                    $result = $wpdb->update($table_name, $item, array('id' => $item['id']));
                    if ($result) {
                        $message = __('Item was successfully updated', 'amarre');
                    } else {
                        $notice = __('There was an error while updating item' + $result, 'amarre');
                    }
                }
            } else {
                // if $item_valid not true it contains error message(s)
                $notice = $item_valid;
            }
        }
        else {
            // if this is not post back we load item to edit or give new one to create
            $item = $default;
            if (isset($_REQUEST['id'])) {
                $item = $wpdb->get_row($wpdb->prepare("SELECT * FROM $table_name WHERE id = %d", $_REQUEST['id']), ARRAY_A);
                if (!$item) {
                    $item = $default;
                    $notice = __('Item not found', 'amarre');
                }
            }
        }