The immediate workaround to this problem is to open up a new SSMS session, then navigate to the SQL Agent Job you want to edit. At that point, you will be able to make the edit and save the changes.
For more information on why this error occurs and the possible release(s) fixes, check out this url:
http://connect.microsoft.com/SQLServer/feedback/details/557402/ssms-can-no-longer-create-or-edit-job-steps