TummyTime
SDK no longer give you a warning about being in play mode when you try to build content. It only throws an error in the logs. This happens in 3.8.1-beta.1 as well.
This one to be more exact -> NullReferenceException: Object reference not set to an instance of an object
  1. In a project, enter playmode.
  2. Click build in the SDK.
  3. check the logs.
Video showing problem: https://youtu.be/EXTIGPi8RxI