The default value needs to be a file ID (the 'managed' part of 'managed_file' means Drupal's taking care of the file for you in the file_managed
table, and any reference to it needs to be via the ID).
If you're currently saving the URL in companyLogo you might want to consider changing that column to an int and saving the file ID instead. That way your existing code will work.
If you need to get the URI at any point you can load the file up:
$uri = file_load($row['companyLogo'])->uri;
And if you need the full URL from that:
$url = file_create_url($uri);