Thanks for the video. I’ve reproduced this and confirmed it’s a bug in Studio’s designer.
Because MyContainer is inside MyModule, the generated window code needs to refer to it as MyModule.MyContainer. Studio currently drops the module name, causing the two compiler errors shown in your video.
I’ve logged the bug here, with regression-test requirements for container controls inside modules.
For now, keeping the container control at the project’s top level, outside the module, should avoid the problem.
