To avoid duplicated code, you should modify the constructors to:

```java
 	public ApplicationAssets(Files files) {
                this(files, null);
 	}
        public ApplicationAssets(Files files, String skin){
// ...
       if ( skin != null ){
            setSkin(skin);
      }
     //...
}
```
and also updated javadoc to point out that skin parameter can be null.