When I add a filter to custom get_all_posts API callback, API works, but the admin page for each page is blank. Why?

function get_all_posts( $data, $post, $context ) {

    return [
        'id'        => $data->data['id'],
        'date'      => $data->data['date'],
        'date_gmt'  => $data->data['date_gmt'],
        'modified'  => $data->data['modified'],
        'title'     => $data->data['title']['rendered'],
        'content'   => $data->data['content']['rendered'],
        'main_text' => $data->data['main_text'],
        'featured_media_url'   => $data->data['jetpack_featured_media_url'],
        'category'  => get_the_category_by_ID( $data->data['categories'][0] ),
        'link'      => $data->data['link']
    ];
}

add_filter( 'rest_prepare_post', 'get_all_posts', 10, 3 );

EDIT:
Got this error on the webpage Unhandled Promise Rejection: TypeError: undefined is not an object (evaluating 'e.length')

FYI: I added a meta box on the post admin page, the meta box works perfectly without the code above.

My entire code:

function main_text_meta_box() {

    add_meta_box(
        'main-text',
        __( 'Main text', 'sitepoint' ),
        'main_text_meta_box_callback',
        'post'
    );
}

function main_text_meta_box_callback( $post ) {

    // Add a nonce field so we can check for it later.
    wp_nonce_field( 'main_text_nonce', 'main_text_nonce' );

    $value = get_post_meta( $post->ID, '_main_text', true );

    echo '<textarea style="width:100%; height:300px;" id="main_text" name="main_text">' . esc_attr( $value ) . '</textarea>';
}

add_action( 'add_meta_boxes', 'main_text_meta_box' );


function save_main_text_meta_box_data( $post_id ) {

    // Check if our nonce is set.
    if ( ! isset( $_POST['main_text_nonce'] ) ) {
        return;
    }

    // Verify that the nonce is valid.
    if ( ! wp_verify_nonce( $_POST['main_text_nonce'], 'main_text_nonce' ) ) {
        return;
    }

    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }

    // Check the user's permissions.
    if ( isset( $_POST['post_type'] ) && 'page' == $_POST['post_type'] ) {

        if ( ! current_user_can( 'edit_page', $post_id ) ) {
            return;
        }

    }
    else {

        if ( ! current_user_can( 'edit_post', $post_id ) ) {
            return;
        }
    }

    /* OK, it's safe for us to save the data now. */

    // Make sure that it is set.
    if ( ! isset( $_POST['main_text'] ) ) {
        return;
    }

    // Sanitize user input.
    $my_data = sanitize_text_field( $_POST['main_text'] );

    // Update the meta field in the database.
    update_post_meta( $post_id, '_main_text', $my_data );
}
add_action( 'save_post', 'save_main_text_meta_box_data' );

function add_custom_fields() {
    register_rest_field(
         'post', 
         'main_text', //New Field Name in JSON RESPONSEs
         array(
          'get_callback'    => 'get_main_text', // custom function name 
          'update_callback' => null,
          'schema'          => null,
        )
    );
}
add_action( 'rest_api_init', 'add_custom_fields' );


function get_main_text ( $post, $field_name, $request ){

        return get_post_meta($post['id'], '_main_text', true);
}