fix selection tray collider

[mikeskydev]

Fix an issue with the selection tray prefab, seems to have been damaged somehow.

[mikeage]

@mikeskydev , we have an (automatic) PR (#505 ) to update the github action we use for building, which is failing on a prefab error:

Start importing Assets/Prefabs/UIComponents/SelectionTray.prefab using Guid(9b674e23f212f5a40b5d981d3ba5f4e2) Importer(-1,00000000000000000000000000000000) Problem detected while importing the Prefab file: 'Assets/Prefabs/UIComponents/SelectionTray.prefab'.
The file might be corrupt or have missing nested Prefabs. See details below.
Errors:
	Restored Transform child parent pointer from NULL
 -> (artifact id: '16e0ea62ac9a5abac71aa778cef9da71') in 0.009217 seconds 

Is that fixed by this PR?

I think it might only show up there because the Library/ cache is not valid and so it has to re-import everything. Maybe.

[mikeskydev]

Yes! I did a fresh clone yesterday and noticed this same error, so it should fix it

[mikeage]

Great, thanks! Merge away?