I've just solved a problem which I wanted to share so other people won't loose hours/days of searching.
I'm using Flash CS6, SDK 4.6-AIR 3.3 inside FDT on OSX
I've used WCK for quite a while now and delivered a few projects with 'great success'.
But recently I encountered a weird problem.
Everything went fine for a while in my current project. But suddenly I got this error: "Cannot access a property or method of a null object reference
" when compiling from the SDK. Flash compiled my .swc, which contained the wck.World file just fine. But the SDK threw up. I checked the wck.World-world I build inside my .swc and found nothing special. Now I traced my steps back and I found out that I used the new option introduced in Flash CS5.5 (Cache-as-bitmap). I know it's been there for quite a while in code but never in the IDE. To make it short, Don't use the option 'Cache as Bitmap' inside the flash authoring tool for graphical-assets that extend ShapeBase. Sometimes it goes ok, sometimes it doesn't. Flash will give you no error when compiling the .swc but if you put the level somewhere on the stage the compiler will spit out the above error.
Just an F.Y.I. for everyone who encounters this problem.