Wait method in Tree Item discovering is more stable now

Supposing Tree items are generated after parent Tree Item expansion. Let's say first child Tree Item is generated as first one and the second child Tree Item as second one.

With current approach there is waiting until at least one child in Tree Item is found. In our case it would stop after first child Tree Item discovering. Problem is, that if we wanted the second child Tree Item, it will be not found cause it is generated later then the first one so children discovering process returns only one child not two children Tree Items.