ArgumentNullException in Timeline Unity v2017.2.0b5

Unity v2017.2.0b5

Googled for this particular Stacktrace. Saw others with the same message but different stacktraces.

Last non .Net function:

UnityEditor.Timeline.TimelineWindow+TimelineState.ArmForRecord (UnityEngine.Timeline.TrackAsset track) (at C:/buildslave/unity/build/Extensions/Timeline/Editor/Window/TimelineState.cs:894)
UnityEditor.Timelin


Included here in case it helps.

In Full

ArgumentNullException: Value cannot be null.
Parameter name: key
System.ThrowHelper.ThrowArgumentNullException (System.ExceptionArgument argument) (at <aa1756d827e14afe8f4b9355672b2ab4>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].Insert (TKey key, TValue value, System.Boolean add) (at <aa1756d827e14afe8f4b9355672b2ab4>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].set_Item (TKey key, TValue value) (at <aa1756d827e14afe8f4b9355672b2ab4>:0)
UnityEditor.Timeline.TimelineWindow+TimelineState.ArmForRecord (UnityEngine.Timeline.TrackAsset track) (at C:/buildslave/unity/build/Extensions/Timeline/Editor/Window/TimelineState.cs:894)
UnityEditor.Timeline.TimelineWindowTimeControl.StartRecording (UnityEngine.Object targetObject) (at C:/buildslave/unity/build/Extensions/Timeline/Editor/Window/TimelineWindowTimeControl.cs:268)
UnityEditorInternal.AnimationWindowState.StartRecording () (at C:/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimationWindowState.cs:507)
UnityEditor.AnimEditor.RecordButtonOnGUI () (at C:/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimEditor.cs:893)
UnityEditor.AnimEditor.PlayControlsOnGUI () (at C:/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimEditor.cs:676)
UnityEditor.AnimEditor.OnAnimEditorGUI (UnityEditor.EditorWindow parent, UnityEngine.Rect position) (at C:/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimEditor.cs:188)
UnityEditor.AnimationWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/Animation/AnimationWindow/AnimationWindow.cs:88)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <aa1756d827e14afe8f4b9355672b2ab4>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <aa1756d827e14afe8f4b9355672b2ab4>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <aa1756d827e14afe8f4b9355672b2ab4>:0)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:283)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:276)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:249)
UnityEditor.DockArea.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:383)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:143)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleEvent (UnityEngine.Experimental.UIElements.EventBase evt) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:213)
UnityEngine.Experimental.UIElements.EventDispatcher.DispatchEvent (UnityEngine.Experimental.UIElements.EventBase evt, UnityEngine.Experimental.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/EventDispatcher.cs:182)
UnityEngine.Experimental.UIElements.UIElementsUtility.DoDispatch (UnityEngine.Experimental.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:207)
UnityEngine.Experimental.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:72)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUIUtility.cs:175)