Looking at the following code:
$image_meta_data = wp_get_attachment_metadata( $this->id );
…
why do you expect to see ‘full’ parent image-size in the array of sub-sizes?

// Fix E_WARNING: Illegal string offset 'width'
        if(!is_array($image_meta_data['sizes']['full'])) {
            $image_meta_data['sizes']['full'] = array();
        }

The standard wp_get_attachment_metadata() output looks like this:

Array
(
    [width] => 2400
    [height] => 1559
    [file] => 2011/12/press_image.jpg
    [sizes] => Array
    (
        [thumbnail] => Array
        (
            [file] => press_image-150x150.jpg
            [width] => 150
            [height] => 150
            [mime-type] => image/jpeg
        )
        [medium] => Array
        (
            [file] => press_image-4-300x194.jpg
            [width] => 300
            [height] => 194
            [mime-type] => image/jpeg
        )
        [large] => Array
        (
            [file] => press_image-1024x665.jpg
            [width] => 1024
            [height] => 665
            [mime-type] => image/jpeg
        )
        [post-thumbnail] => Array
        (
            [file] => press_image-624x405.jpg
            [width] => 624
            [height] => 405
            [mime-type] => image/jpeg
        )
    )
    [image_meta] => Array
    (
        [aperture] => 5
        [credit] => 
        [camera] => Canon EOS-1Ds Mark III
         => 
        [created_timestamp] => 1323190643
        [copyright] => 
        [focal_length] => 35
        [iso] => 800
        [shutter_speed] => 0.016666666666667
        [title] => 
    )
)

https://developer.wordpress.org/reference/functions/wp_get_attachment_metadata/

• This reply was modified 1 year, 9 months ago by Ivan Privalov.
• This reply was modified 1 year, 9 months ago by Ivan Privalov.

Try to dump the array with:

        $image_meta_data = wp_get_attachment_metadata( $this->id );

        echo '<pre>';
        var_dump($image_meta_data);
        echo '</pre>';

Hello Stefan, thank you for fixing this in the front-end.
Please, notice, as I said earlier, that the issue also exists in the back-end in the plugin setting page – /wp-admin/options-general.php?page=responsify-wp

Screenshot: https://i.imgur.com/UIfOzvh.png

Notice: Undefined index: full in /wp-content/plugins/responsify-wp/admin/views/selected_sizes.php on line 23 and 24

Yes, Stefan, the new version fixed the ‘notice’ issues in the front-end and the back-end for our website.

Thank you a lot!