Error Condition: Cannot insert the value null into column ‘diagram_id’, table dbo.sysdiagrams
Problem: This sometimes occurs when a SQL Server database is ported from one version to another (such as 2000 to 2005 or 2005 to 2008) or possibly also when using the SSMS for a newer version of SQL Server against and older database.
Fix: Drop and recreated dbo.sysdiagrams making the column diagram_id an identity column.
CREATE TABLE [dbo].[sysdiagrams]
[principal_id] [int] NOT NULL,
[diagram_id] [int] identity(1,1) NOT NULL,
[version] [int] NULL,
[definition] [varbinary](max) NULL
NOTE: If you want to preserve the diagrams, then rename the table and create a new one, then SELECT – INTO to copy the data into the new table.