Hierarchy does not display nodes when loading multiple models

[jiajasper]

To reproduce this issue

1. Prepare three models (ideally very different models in terms of size so you can see the models being loaded into the scene)
2. Load the first model in
3. Drag and drop the second model in while holding shift

You should now see that the second model is loaded in the scene but the nodes are not showing up in the hierarchy panel.

4. Drag and drop the third model in while holding shift

The nodes for the second model will now show up in the hierarchy but not the third model's nodes.

We can keep adding models in but the hierarchy will only show nodes for the previous models that are dragged into the scene - the latest model will be displayed in the scene but its nodes will not show in the hierarchy.

In the image below, I had 3 models loaded into the scene but only 2 are displayed in the hierarchy.

[slimbuck]

Might be related to https://github.com/playcanvas/model-viewer/pull/162