NullReferenceException error thrown while dragging animation clip on track group in Timeline window.

Steps to repro:
1. Open MultipleClips scene from the attached project.
2. Windows > Timeline.
3. Select Cube game object.
4. Select "Recorded" animation clip and extend it.
5. Try to drag and drop "Recorded" animation clip on Track group section below.

Observe the Exceptions in the console window.

Reproducible on:
2018.2.0a7, 2018.2.0a6, 2018.2.0a4, 2018.2.0a3, 2018.2.0a2.

Working fine on:
2018.2.0a1, 2018.1.0b13.

Environment:
Occurring on Windows 10, Windows 7, Mac 10.13, Mac 10.12.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Timeline.ClipInspector.OnHeaderIconGUI (Rect iconRect) (at C:/buildslave/unity/build/Extensions/Timeline/Editor/inspectors/ClipInspector/ClipInspector.cs:191)
UnityEditor.Editor.DrawHeaderGUI (UnityEditor.Editor editor, System.String header, Single leftMargin) (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:803)
UnityEditor.Editor.DrawHeaderGUI (UnityEditor.Editor editor, System.String header) (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:772)
UnityEditor.Editor.OnHeaderGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:662)
UnityEditor.Editor.DrawHeader () (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:655)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1169)
UnityEditor.InspectorWindow.DrawEditors (UnityEditor.Editor[] editors) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1073)
UnityEditor.InspectorWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:413)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:295)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:288)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:255)