On selecting "Restore Default Pose" property under "CCD Solver 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 > CCD Solver 2D (Script)
5. Add any Gameoject in Effector Property
6. Change Chain Length slider value
7. Select "Restore Default Pose" Button.

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

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

Tested with Unity versions:
2020.1.0a19, 2019.3.0f6 (3805be3cb06a)

Package found version: preview.1 - 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
UnityEditor.Experimental.U2D.IK.IKEditorManager.DoUndo (UnityEngine.Experimental.U2D.IK.IKManager2D manager, System.String undoName, System.Boolean record) (at Library/PackageCache/com.unity.2d.ik@3.0.0-preview.1/Editor/IKEditorManager.cs:161)
UnityEditor.Experimental.U2D.IK.IKEditorManager.Record (UnityEngine.Experimental.U2D.IK.Solver2D solver, System.String undoName) (at Library/PackageCache/com.unity.2d.ik@3.0.0-preview.1/Editor/IKEditorManager.cs:139)
UnityEditor.Experimental.U2D.IK.Solver2DEditor.DoRestoreDefaultPoseButton () (at Library/PackageCache/com.unity.2d.ik@3.0.0-preview.1/Editor/Inspectors/Solver2DEditor.cs:107)
UnityEditor.Experimental.U2D.IK.Solver2DEditor.DrawCommonSolverInspector () (at Library/PackageCache/com.unity.2d.ik@3.0.0-preview.1/Editor/Inspectors/Solver2DEditor.cs:51)
UnityEditor.Experimental.U2D.IK.CCDSolver2DEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.2d.ik@3.0.0-preview.1/Editor/Inspectors/CCDSolver2DEditor.cs:54)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <b9bb103d86cd4249b6a1c8b6aa2850c7>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)