NullReferenceException error is thrown on assigning unity-font to none in Visual Element section.

Steps to repro:
1. Create a new project.
2. Windows > Analysis> UIElement Debugger.
3. Select ProjectBrowser window from Select Panel dropdown.
4. Select VisualElement > Click on Show All > Assign unity-font to Arial.
5. Assign unity-font to none.

Actual Result:
NullReferenceException error is thrown in the console.

Expected Result:
No error in the console.

Reproducible in:
2020.1.0a2, 2019.3.0b1, 2019.3.0a1.

Working Fine in:
2019.2.0f1.

Environment:
Windows & Mac 10.14.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.UIElements.Debugger.StyleField.SetPropertyValue (System.Object newValue) (at <ed82b0b80ec5457da3c2e9cdf90adc95>:0)
UnityEditor.UIElements.Debugger.StyleField.<GetOrCreateField>b__9_0[T,U] (UnityEngine.UIElements.ChangeEvent`1[T] e) (at <ed82b0b80ec5457da3c2e9cdf90adc95>:0)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventCallbackRegistry.InvokeCallbacks (UnityEngine.UIElements.EventBase evt) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEvent (UnityEngine.UIElements.EventBase evt) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatchUtilities.PropagateEvent (UnityEngine.UIElements.EventBase evt) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.DefaultDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <3dd62d3208914b14a9c3250b5037e105>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <47533d5f70c041debc5c751474c7409d>:0)