Widget Blueprint could not be loaded because it derives from an invalid class 

The blueprint control could not be loaded because it is from an invalid class

To generate node widgets a class must be created

This is my problem when packaging a project with a custom plugin

The reason for the problem: When the compiler is opened, the plug-in is loaded by default after the compiler is loaded.

Solution:

In the .uplugin file put

" LoadingPhase " : " Default "

changed to

" LoadingPhase " : " PreDefault "

It means that the loading order of the plug-ins is mentioned before the compiler is loaded, so that this problem can be solved.