Array to string conversion in Drupal\Core\Theme\ThemeInitialization->getActiveTheme()

I get this error:

Notice: Array to string conversion in Drupal\Core\Theme\ThemeInitialization->getActiveTheme() (line 179 of core/lib/Drupal/Core/Theme/ThemeInitialization.php).

$theme->info['logo'] for some themes is empty even if the logo is set to a custom path, for others is an array, but never a string.