[Animation] Null Reference exception is thrown on moving threshold slider in blend tree

Steps to Repro:
1. Create a new Project
2. Project window > right click
3. Animator Controller
4. Double click on Animator controller
5. Animator window > right click
6. Create state > blend tree
7. Double click on blend tree
8. Select blend tree and click '+'
9. Add two motion fields
10. Disable automate thresholds
11. Set all threshold value to 0
12. Move red slider

Actual Result:
Null Refrence excpetion occurs

Expected Result:
The slider moves without any errors or warning and given to the user

Environment:
Windows and Mac

Occurring with:
2019.3.0a2, 2019.2.0b2, 2019.1.0f2, 2018.3.11f1, 2017.4.7f1

Stack Trace:
Error1:
Retrieving array element that was out of bounds
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Error2:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.BlendTreeInspector.BlendGraph (UnityEngine.Rect area) (at C:/buildslave/unity/build/Editor/Mono/Inspector/BlendTreeInspector.cs:673)
UnityEditor.BlendTreeInspector.OnInspectorGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/BlendTreeInspector.cs:327)
UnityEditor.UIElements.InspectorElement+<CreateIMGUIInspectorFromEditor>c__AnonStorey1.<>m__0 () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorElement.cs:479)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)