Skin for * cannot be found

What caused the error?
I was moving one of my modular applications from Flex 3 (Gumbo) to Flex 4 (Spark) architecture. This error happend each time I added a module GUI to the main application. Unfortunately this is all what I found out about this problem.

What is the solution?
Add: -keep-all-type-selectors to your compiler arguments.

Here’s is what documentation says about this argument:
“Instructs the compiler to keep a style sheet’s type selector in a SWF file, even if that type (the class) is not used in the application. This is useful when you have a modular application that loads other applications. For example, the loading SWF file might define a type selector for a type used in the loaded (or, target) SWF file. If you set this option to true when compiling the loading SWF file, then the target SWF file will have access to that type selector when it is loaded. If you set this option to false, the compiler will not include that type selector in the loading SWF file at compile time. As a result, the styles will not be available to the target SWF file.

This is an advanced option.”