设计模式不适用于Flex 4项目

I'm trying to port a small Flex 3 AIR project to Flex 4 (Gumbo). Things are working quite fine. I can run and debug the application. But for some reason I can't use the design mode of my WindowedApplication. The head of the MXML file looks like so:I'm trying to port a small Flex 3 AIR project t