Reproduction steps:
1. Create a terrain and open terrain inspector
2. Go to "Add trees" tab
3. Press "Edit Trees -> Add tree"
4. In the pop-up window you can only add one object as a tree
5. If there are multiple trees added, you cannot delete all of them. You have to delete them one by one.

Note, that these are usability issues. When working with terrain it is very time consuming to add and delete trees one by one