As a programmer, seeing the error message “Object reference not set to an instance of an object” is pretty common and pretty annoying.  What’s even more annoying is when you get that error when simply trying to get the latest source code from your Team Foundation Server within Visual Studio, as shown below.

Source Control Explorer Error

At least in my case, this error resulted from the primary hard drive on the TFS server being completely full.   TFS 2015 recommends at least 50 GB of free space be available on the drive where TFS is installed.