You can't delete a blend tree when inside the blend tree node. If you try, a pop up appears "Are sure you want to delete". If you select delete nothing happens. If you do the same outside of the blend tree you don't get a pop-up and the blend tree is deleted.

Expected
1. You should not have the option to delete the blend tree root node from inside. If there is a pop up it should state this.
2. If you want to add some validation on deleting a blend tree from outside (Blend Layer) you should include the pop up there.

Steps to reproduce
- Open the attached project in 2018.3.0f2
- Open the Animator window.
- Right-click in the scene and add a "Create State > From New Blend Tree"
- Double click on the new blend tree to open it.
- Select the blend tree and press delete
- A popup appears, press delete.
Notice Issue >> Nothing happens.

NOTE
I know this pop up is for when you add more blending nodes to the root node, however, you should not get the prompt on the root node.

Tested: 2019.1.0a11, 2018.3.0f2, 2017.1.5f1