TFS client error "object reference not set to an instance of an object ."

Recently, when a codeplex project is opened, this error always occurs: "object reference not set to an instance of an object .". I open a project through "file-> source control-> open from sourcecontrol. Other projects can be opened, but one project has always encountered this problem.

 

I also got this error when I opened it: the path <local path> is already mapped in Workspace <machine name [old TFs server]>. I Googled it and found this article.Article: For TFs workspace error, refer to the methods in the preceding section. Open cmd of vs2008 and run the "TF workspace/Delete [/Server: servername] workspacename [; workspaceowner]" command to delete the workspace of a specific project, solve the problem.