Visual inheritance was working just fine until now. The designer shows the parent form OK but the inherited ones gave me this error:
System.Windows.Forms.Design.IEventHandlerService service already exists in the service container. Parameter name: serviceType
Now the designer has changed its message and what I get is "Object reference not sent to an instance of an object". I opened the task list to see in which file the error was and there's nothing in the file column of the list. If I double click the task it just places me on the first line of code in the file where the form is declared.
The application still works fine though so I guess this must be an error with Visual Studio and not with my code.