Database diagram - Attempted to read or write protected memory

A possible cause is having VS 2012 installed, installing VS 2013 and later uninstalling VS 2013. It leaves the VS 2012 installation corrupted.

Delete: C:\Program Files (x86)\Common Files\microsoft shared\Visual Database Tools\dsref80.dll
and everything in C:\Program Files (x86)\Common Files\microsoft shared\MSDesigners8\, then run a repair for VS 2012

Thanks to stackoverflow