How to reproduce:
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 Motion" from the context menu
9. Press Ctrl+z or go to Edit -> "Undo Added BlendTree Child"

Expected result: the empty animation is removed from the list
Actual result: the last animation in the list is removed i.e. "AnimationTwo"

Reproducible with: 2018.3.9f1, 2019.1.0b7, 2019.2.0a8
Not reproducible with: 2017.4.23f1

Note: step 7 is necessary for the problem to reproduce (the focus can be shifted anywhere as long as it goes off the Inspector). Also, that "Automate Threshold" checkbox in the BlendTree's inspector has to be turned off (it is only present with two or more motions in the list)