Visual Studio 2012 - Operation is not valid due to the current state of the object.

Problem: 

When I open a package in VS 2012, I get an error message on the dialog box

"Operation is not valid due to the current state of the object." 

Then I try to open/create a project instead, I get the same error message.

Solution:  

In Control Panel, find "SQL Server Data Tools for Visual Studio 2012", choose repair (not uninstall). Fixed!