On Selecting "Reset Curve" Button in StreamingImageSequence causes the editor to throw NullReferenceException, refer to the attached video.

Steps to repro:
1. Create a new project
2. Windows > Package Manager > Streaming Image Sequence > Install
3. Project > right-click > Streaming Image Sequence > Click Clip
4. Open File Window > Select any image > Open
5.Streaming Image Sequence Importer > OK
5. Project > StreamingImageSequence > Select Reset Curve

Actual Result:
NullReferenceException is thrown on selecting "Reset Curve" Button in StreamingImageSequence.

Expected Result:
No errors in the console on selecting "Reset Curve" Button

Reproducible in:
2020.2.0a15, 2020.1.0b14, 2019.3.14f1,with Package Version 0.2.0, 0.1.5 , 0.1.4
2019.3.0a1 with Package Version 0.1.4

Note: StreamingImageSequence package is broken in Package version 0.1.3

Environment:
Windows 10 & macOS 10.14

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.StreamingImageSequence.StreamingImageSequencePlayableAsset.ValidateAnimationCurve (UnityEngine.AnimationCurve& animationCurve) (at Library/PackageCache/com.unity.streaming-image-sequence@0.2.0-preview/Runtime/PlayableAssets/StreamingImageSequence/StreamingImageSequencePlayableAsset.cs:648)
UnityEngine.StreamingImageSequence.StreamingImageSequencePlayableAsset.ResetAnimationCurve () (at Library/PackageCache/com.unity.streaming-image-sequence@0.2.0-preview/Runtime/PlayableAssets/StreamingImageSequence/StreamingImageSequencePlayableAsset.cs:616)
UnityEditor.StreamingImageSequence.StreamingImageSequencePlayableAssetInspector.OnInspectorGUI () (at Library/PackageCache/com.unity.streaming-image-sequence@0.2.0-preview/Editor/StreamingImageSequence/StreamingImageSequencePlayableAssetInspector.cs:80)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <47f38d2e628840eaa536de5b3e34149b>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)