In seoGetimage function in seo.php, line 78 creates the error Call to a member function media() on null if the page of image is not found:

$imgarray = $this->grav['page']->find($fixedurl)->media()->images();

To fix this, we need to check if the page exists or not first, something like this:

$page = $this->grav['page']->find($fixedurl);

if (!$page)
    return [
        'url'       => '',
        'width'     => '',
        'height'    => '',
    ];

$imgarray = $page->media()->images();

Because I am not familiar with this plugin so I don't know if returning empty URL, width and height would effect somewhere else or not, so I don't submit a pull request for this, at least with this simple fix the error is gone and the site's running again.