On selecting "Restore Default Pose" property under "IK Manager 2D(Script)" causes the editor to throw NullReferenceException, refer attached video.

Steps to repro:
1. Create a new Project
2. Windows > Package Manager > 2D IK > Install
3. Hierarchy > right-click > Create Empty
4. Inspector > Add Component > Limb Solver 2D (Script)
5. Inspector > Add Component > IK Manager 2D (Script)
6. Select "Restore Default Pose" Button.

Actual Result:
NullReferenceException is thrown on selecting "Restore Default Pose" Property under "IK Manager 2D (Script)"

Expected Result:
Restore Default Pose property is selected without any errors.

Tested with Unity versions:
2020.2.0a1, 2020.1.0a25, 2019.3.3f1, 2018.1.0b13

Package found version: preview.2 - 3.0.0, preview.1 - 2.0.0, preview.1 - 1.3.0, preview.1 - 1.0.5,

Environment:
Windows10 and macOS 10.14

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.Experimental.U2D.IK.IKChain2D.RestoreDefaultPose (System.Boolean targetRotationIsConstrained) (at library/packagecache/com.unity.2d.ik@3.0.0-preview.2/runtime/ikchain2d.cs:151)
UnityEditor.Experimental.U2D.IK.IKManager2DEditor.DoRestoreDefaultPoseButton () (at library/packagecache/com.unity.2d.ik@3.0.0-preview.2/editor/inspectors/ikmanager2deditor.cs:159)
UnityEditor.Experimental.U2D.IK.IKManager2DEditor.OnInspectorGUI () (at library/packagecache/com.unity.2d.ik@3.0.0-preview.2/editor/inspectors/ikmanager2deditor.cs:136)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <e859a479319a4a5a8d9711977e98d28e>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)