you are trying to add a mesh to the scene which is already in the scene and you are also using 2 scenes in the playground,
I reorganized a bit the code here:
https://playground.babylonjs.com/#ZPY65X
I want to add the imported mesh to the current scene or mesh child.
Hey there, you can use ImportMesh to do this, like below. 
This will be added to the scene immediately.I want to store it in the root mesh first,then render it.
https://playground.babylonjs.com/#WGZLGJ#4030
Ah, I get what you’re after now - it looks like you’ve already found a way, but you can also do it by simply calling assetContainer.instantiateModelsToScene() 
https://playground.babylonjs.com/#WGZLGJ#4034
Here’s the relevant documentation section for doing it this way. 
Thanks,i get it