How to reproduce:
1. Open attached project "case_1028113-BlendTreeBug.zip" and scene "ikTestScene"
2. In Hierarchy window, select "ikTestChar" object
3. In Animator window, double-click on "idle" state
4. In Animator window, select "testBlendtree1" blend tree
5. In Project window, drag "Assets" -> "BlendTrees" -> "testBlendtree4" blend tree into one of the motion tabs in Inspector window

Expected result: blend tree is changed without casualties
Actual result: a "Changing BlendTree will delete previous BlendTree" message box appears, pressing "Delete" results in a corruption of the overwritten blendtree asset

Reproducible with: 5.6.6f2, 2017.1.3p3, 2017.2.2p4, 2017.4.2f2, 2018.1.0f2, 2018.2.0b2