If you’ve been using the AssetUrlSelector for SharePoint the following may help if you’ve recently started getting the following error in the ULS logs and are not able to view list items.
Error while executing web part: System.NullReferenceException: Object reference not set to an instance of an object.
The cause of this error is likely due to a recent update applied to the server. The most likely candidate is the recent KB update – 2844287 – MS13-052: Description of the security update for the .NET Framework 2.0 Service Pack 2 on Windows Vista Service Pack 2 and Windows Server 2008 Service Pack 2: July 9, 2013
Microsoft have recently released a hotfix for this issue (and a few others). The hotfix can be downloaded from the Microsoft site here.
I can confirm that after applying this hotfix, the assert URL selector dialog displayed list items without generating an error.