Problem:
When you create a new project or upgrade an old one to Sitefinity 4.2. 1650 and edit it in Visual Studio in order to add custom modules /custom controls you may get this error:
Causes and Solution:
The error is thrown because of a missing reference to Telerik.Sitefinity.MigrationContracts.dll. This assembly is removed from your project references when you Clean or Rebuild a project containing Telerik.Sitefinity.MigrationContracts.dll or the entire solution.
To fix the problem add a reference to the missing assembly Telerik.Sitefinity.MigrationContracts from a blank Sitefinity 4.2 project or from _EmptyProject located in the folder where Sitefinity 4.2 project manager is placed.
In order to avoid getting this error again make sure not to clean or rebuild the projects which reference this assembly. You can use Build project option instead.