Handle Missing Parent in deleteAction

[cancan101]

Similar issue to #800

This line can return null:

admin-ui-classic-bundle/src/Controller/Admin/Asset/AssetController.php

Line 671 in 2013cc6

$parentAsset = Asset::getById((int) $request->get('id'));

leading to error here:

admin-ui-classic-bundle/src/Controller/Admin/Asset/AssetController.php

Line 674 in 2013cc6

$list->setCondition('`path` LIKE ?', [Helper::escapeLike($parentAsset->getRealFullPath()) . '/%']);

Call to a member function getRealFullPath() on null

I assume this is coming from a race when deleting / renaming folders.