On editing vertices values from custom geometry in shape tool window, editor throws NullReferenceException continuously also on using "Enter" key "shape tool window" get closed, refer attached video.

Steps to repro:
1. Create a new project.
2. Window > Package Manager > ProBuilder (4.1.2) > Install
3. Tools > ProBuilder > ProBuilder Window
4. New Shape > Click on [+] button > Shape Tool
5. Shape Selector > Custom > Custom Geometry
6. Edit Vertices values

Actual Result:
NullReferenceException is thrown continuously in the console while editing vertices values

Expected Result:
Custom geometry vertices values get edited gracefully without throwing any exception

Reproducible in:
2020.1.0a15, 2019.3.0f2

Probuilder package version found: 4.2.1, 4.1.0, 4.0.5

Package working fine version: 4.0.4, 3.10.1, 3.0.0

Environment:
Windows 10 & macOS 10.14.

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ProBuilder.ShapeEditor.SetPreviewMesh (UnityEngine.ProBuilder.ProBuilderMesh mesh) (at library/packagecache/com.unity.probuilder@4.1.2/editor/editorcore/shapeeditor.cs:172)
UnityEditor.ProBuilder.ShapeEditor.OnGUI () (at library/packagecache/com.unity.probuilder@4.1.2/editor/editorcore/shapeeditor.cs:126)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <437ba245d8404784b9fbab9b439ac908>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <437ba245d8404784b9fbab9b439ac908>:0)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)
UnityEditor.HostView.Invoke (System.String methodName) (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)
UnityEditor.HostView.OldOnGUI () (at <1df4bfcafa864ca99de2ab91a95ebbc4>:0)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Boolean canAffectFocus) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Boolean canAffectFocus) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, System.Boolean canAffectFocus) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.IMGUIContainer.SendEventToIMGUIPrivate (UnityEngine.UIElements.EventBase evt, System.Boolean canAffectFocus) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.IMGUIContainer.SendEventToIMGUI (UnityEngine.UIElements.EventBase evt, System.Boolean canAffectFocus) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.KeyboardEventDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <43e6943a13fc4b8a95e64862fae6571b>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <023540da4c6041d9a2d1762b4d9d517c>:0)