Null Reference Exception occurs on editing cloth constraints or collision. Once the exception occurs the editing window doesn't open. Refer the video attached

Steps to reproduce:
1) Create new project > Create empty gameObject.
2) Add cloth component to empty GameObject.
3) Click on Edit Cloth constraint /Cloth self-collision image button and press select in the editing window.

Actual result: Null reference exception occurs and the cloth editing window doesn't open anymore
Expected result: No exception occurs and the cloth editing window works as expected.

Environment:
Windows and Mac

Occurring with:

2018.3.0a3, 2018.2.0b10, 2018.1.0b1, 2017.3.03f1

Workaround : Select any mesh in the mesh renderer so that the errors disappear and then follow step 3 in steps to repro.

Stack Trace
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ClothInspector.SelectionGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/ClothInspector.cs:726)
UnityEditor.ClothInspector.ConstraintEditing (UnityEngine.Object unused, UnityEditor.SceneView sceneView) (at C:/buildslave/unity/build/Editor/Mono/Inspector/ClothInspector.cs:1598)
UnityEditor.SceneViewOverlay.WindowTrampoline (Int32 id) (at C:/buildslave/unity/build/Editor/Mono/SceneView/SceneViewOverlay.cs:112)
UnityEngine.GUILayout+LayoutedWindow.DoWindow (Int32 windowID) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:517)
UnityEngine.GUI.CallWindowDelegate (UnityEngine.WindowFunction func, Int32 id, Int32 instanceID, UnityEngine.GUISkin _skin, Int32 forceRect, Single width, Single height, UnityEngine.GUIStyle style) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUI.cs:1675)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)