Undefined index: field_license

variables: a:6:{s:5:"%type";s:6:"Notice";s:8:"!message";s:30:"Undefined index: field_license";s:9:"%function";s:26:"media_gallery_form_alter()";s:5:"%file";s:76:"/var/www/drupal/sites/all/modules/contrib/media_gallery/media_gallery.module";s:5:"%line";i:830;s:14:"severity_level";i:5;}
 count(*): 1370

The logic in media_gallery_form_alter() expects the field_license to be there. That is not the case if the field instance has been removed form the file entity.