Steps to reproduce the issue

1. Create new banner
2. Put a title
3. Choose a client.
4. Save.

Be aware that the problem only occurs on new banner, and if you choose a client.

Expected result

To save a new banner on the selected client.

Actual result

An error has occurred.
0 Call to a member function load() on boolean

System information (as much as possible)

Joomla v4alpha5

Additional comments

The problem is on file /administrator/components/com_banners/Table/BannerTable
line 204. Change from:

$client = Table::getInstance('Client', NAMESPACE . '\', array('dbo' => $db));
to
$client = new ClientTable($db);
I will also try to submit PR that fixes this issue.