This is not the first time. I have seen this error message in the past (probably multiple times), and every time I look to this error and I do not understand, why it appeared and how to fix it. I read the text, projects not referenced, projects have been built… wait… yes, projects must be built, otherwise Inherited Forms can not be shown. Building the projects with base classes solved the problem.
Visual Studio Form Editor: To prevent possible data loss before loading the designer, the following errors must be resolved: The designer could not be shown for this file because none of the classes within it can be designed. The designer inspected the following clasess in the file: MyClass.cs — The base class MyBaseClass.cs could not be loaded. Ensure the assembly has been referenced and that all projects have been built.