Index was outside the bounds of the Array. (Microsoft.SqlServer.smo)

For me this problem still exists with SSMS version 2016 (13.0.16100.1).

A decent workaround is to not use the 'Right click' -> 'Add table...' dialog, but simply drag the table you want to add from the Object Explorer, onto the Diagram surface. While dragging the mouse icon changes into an 'add' symbol and the table is added when you release the mouse.

Beats having to close SSMS every time.

Upgrade your SqlServer management studio from 2008 to 2012

Or Download the service packs of SqlServer Management Studio and update probably resolve you solution

You can download the SQL Server Management studio 2012 from below link

Microsoft® SQL Server® 2012 Express http://www.microsoft.com/en-us/download/details.aspx?id=29062

Restarting the Management Studio worked for me.