After I upgraded vSphere vCenter & Client to 4.1 258902, I been getting an error in the vSphere Client.

A internal error occured in the vSphere Client. Details: Object reference not set to an instant object.

The solution is provided by VMware.
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1033560

It’s just replacing a DLL file 🙂

The error is only with version 4.1 258902 and not 4.1 Update 1.