On selecting the "Bevel" tool from the "ProBuilder" Window while editing shape throws ArgumentNullException, refer to the attached video.

Steps to repro:
1. Create a new project.
2. Window > Package Manager > ProBuilder > Install
3. Tools > ProBuilder > ProBuilder Window
4. Create a "Stair Shape" in the scene view using "NewShape" tool
5. Hierarchy > Stair > Scene > Select any face >ProBuilder Window > Detach faces tool
6. Hierarchy > Stair1 > right-click > Delete object
7. Hierarchy > Stair > Scene > Select any face >ProBuilder Window > Bevel

Actual Result:
ArgumentNullException is thrown

Expected Result:
Shape get edited without throwing any exception

Tested on Unity version:
2020.1.0a24, 2019.3.2f1, 2018.1.0b13

Package version occurring: 4.3.0-preview.2, 4.2.1, 4.1.2, 4.0.2, 3.0.0

Environment:
Windows 10 and macOS 10.14.

Stack trace:
ArgumentNullException: Value cannot be null.
Parameter name: source
System.Linq.Enumerable.Select[TSource,TResult] (System.Collections.Generic.IEnumerable`1[T] source, System.Func`2[T,TResult] selector) (at <351e49e2a5bf4fd6beabb458ce2255f3>:0)
UnityEngine.ProBuilder.MeshOperations.Bevel.BevelEdges (UnityEngine.ProBuilder.ProBuilderMesh mesh, System.Collections.Generic.IList`1[T] edges, System.Single amount) (at library/packagecache/com.unity.probuilder@4.3.0-preview.2/runtime/meshoperations/bevel.cs:202)
UnityEditor.ProBuilder.Actions.BevelEdges.DoAction () (at library/packagecache/com.unity.probuilder@4.3.0-preview.2/editor/menuactions/geometry/beveledges.cs:70)
UnityEditor.ProBuilder.MenuAction.DoButton (System.Boolean isHorizontal, System.Boolean showOptions, UnityEngine.Rect& optionsRect, UnityEngine.GUILayoutOption[] layoutOptions) (at library/packagecache/com.unity.probuilder@4.3.0-preview.2/editor/editorcore/menuaction.cs:337)
UnityEditor.ProBuilder.EditorToolbar.OnGUI () (at library/packagecache/com.unity.probuilder@4.3.0-preview.2/editor/editorcore/editortoolbar.cs:370)
UnityEditor.ProBuilder.ProBuilderEditor.OnGUI () (at library/packagecache/com.unity.probuilder@4.3.0-preview.2/editor/editorcore/probuildereditor.cs:469)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition, UnityEngine.Rect viewRect) (at <ae11075fb7224233bbedf2dc2ef9224c>:0)
UnityEditor.DockArea.DrawView (UnityEngine.Rect viewRect, UnityEngine.Rect dockAreaRect) (at <ae11075fb7224233bbedf2dc2ef9224c>:0)
UnityEditor.DockArea.OldOnGUI () (at <ae11075fb7224233bbedf2dc2ef9224c>:0)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, System.Boolean canAffectFocus) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.IMGUIContainer.SendEventToIMGUIRaw (UnityEngine.UIElements.EventBase evt, System.Boolean canAffectFocus, System.Boolean verifyBounds) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.IMGUIContainer.SendEventToIMGUI (UnityEngine.UIElements.EventBase evt, System.Boolean canAffectFocus, System.Boolean verifyBounds) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.IMGUIContainer.HandleEvent (UnityEngine.UIElements.EventBase evt) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEventAtTargetPhase (UnityEngine.UIElements.EventBase evt) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.MouseCaptureDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.EventDispatcher.ApplyDispatchingStrategies (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, System.Boolean imguiEventIsInitiallyUsed) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <7b3a83bb6c544eadaab9a841ae1beea3>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <fa83d136cc594919bea78eaa8421fde3>:0)