1.Open attached project "case_1134780-AddMotionUndoIssue"
2. Left-click the "Cube" AnimatorController in the Project Window to see it in the Animator Window
3. Double-click the "Blend Tree" node to see its contents
4. Left-click the BlendTree node in its context to see it in the Inspector
5. In the inspector click the "+" symbol and select "Add Motion Field" and select "AnimationOne"
6. Repeat the previous step but with "AnimationTwo" instead of "AnimationOne"
7. Left-Click inside of the Animator Window to set focus
8. Right-click the "BlendTree" node and select "Add New Blend Tree" from the context menu (this one jumps to Threshold 0)
9. Press Ctrl+z or go to Edit -> "Undo Added BlendTree Child"

Expected result: Blend Tree added in step 8 should be removed.

Actual result: "AnimationTwo" is removed.

Reproducible with: 2019.3.0a11, 2019.2.1f1, 2019.1.14f1, 2018.4.6f1, 2018.3.14f1, 2018.2.21f1, 2018.1.9f2

Not reproducible with: 2017.4.31f1