Database diagram support objects cannot be installed because this database does not have a valid owner

When I try to create a new Diagram the following error message is pop up in SQL Server 2012:

Database diagram support objects cannot be installed because this database does not have a valid owner.  To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.

Solution: After running this command this problem was solved. Here TESTDW is a database name.