Only pass the current screen to the list table constructor when we have a...

Only pass the current screen to the list table constructor when we have a screen to work with. This is not the case for admin-ajax requests. fixes #22570.



git-svn-id: https://core.svn.wordpress.org/trunk@22838 1a063a9b-81f0-0310-95a4-ce76da25c4cd