from the log output in your first post, I assume you did not give the ItemEditor instance the id itemEditor, or you did not provide it as an alias in your main.qml file? Please see the StackTheBoxWithEditor demo how to do so.
Also, I would advise not to add a real Platform entity as child of your BuildEntityButton, but only a placeholder image that looks like the platform. Otherwise your entity would be created also in the physics world which is something you probably do not want. Also see the StackTheBoxWithEditor or the Squaby demo for more details how we have done it in real games.
Please let me know if you were able to solve these issues.