Steps to reproduce
1. Open a New Unity Project
2. Create a empty GameObject and select that GameObject
3. Select "Timeline" from "Window" Menu
4. Create a new timeline with Created GameObject
5. Select dropdown icon besides timeline gameobject from Timeline Tab
6. Select " Add from Animation Clip "
7. A popup will come along with null reference exception

Expected Result:
Pop up to select Animation Track should open

Actual Result:
Pop up to select Animation Track opens along with Null reference exception

Error Details:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Timeline.AnimationTrackInspector.OnEnable () (at /Users/builduser/buildslave/unity/build/Extensions/Timeline/Editor/inspectors/AnimationTrackInspector.cs:217)

Frequency:
Always

Reproducible on:
Unity 2018.1.0a4

Not Reproducible on:
Unity 2017.3.0b9 , Unity 2017.2.0f3 ,Unity 2017.1.2f1,Unity 2018.1.0a3

Environment:
Windows and MacOS 10.12
Unity 2018.1.0a5