Reproduction steps:
1. Download project "AnimatorReproProject.zip" and open in Unity
2. Open "Test" scene
3. Add any component to child object Cube (parent Animator Test)
4. Save scene (Ctrl+S)
5. Select object "Animator Test"

Expected: no warnings in Animator Test (Script)
Actual: warnings appear in Animator Test (Script) stating that animator object does not have any parameters

Reproduced on: 5.6.0b11; 5.6.0f1; 5.6.4f1; 2017.1.2p1; 2017.2.0f3; 2017.3.0b5; 2018.1.0a1
Not reproduced on: 5.5.5p1; 5.6.0a1; 5.6.0b10
Regression introduced: 5.6.0b11

Note:
Warnings are generated in AnimatorTargetDrawer script OnGUI().
After entering Play mode works as expected.