It's not possible to save a Draft of a Landing Page that was not published yet.
Steps to reproduce:
1. Create new Landing Page
2. Switch to fields edit mode
3. Fill in name and description
4. Select "Save draft"
Internal Server error with message:
Type error: Argument 1 passed to EzSystems\EzPlatformPageBuilder\Siteaccess\UrlGenerator::getAliasUrl() must be an instance of eZ\Publish\API\Repository\Values\Content\Location, null given, called in /home/vagrant/ee_demo/vendor/ezsystems/ezplatform-page-builder/src/bundle/Controller/PageController.php on line 258 [in vendor/ezsystems/ezplatform-page-builder/src/lib/Siteaccess/UrlGenerator.php:58]