Visual Studio 2015 – Object reference not set to an instance of an object

When i try to access my solution and open one of project files, i get the following error:
Object reference not set to an instance of an object

This solution worked for me (causes a reset of various things like window configurations)

1. Close Visual Studio
2. Logoff / logon
3. Go to folder %AppData%\Microsoft\
4. Rename or delete folder %AppData%\Microsoft\VisualStudio
5. Restart Visual Studio