When no panel is selected in the Panel's dropdown list from the UIElements Debugger window and tried to search the panel by "type, name, or class" in the search box which is at the bottom of the window, causes the Editor to throw NullReferenceException in the Console, Refer attached video

Steps to repro:
1. Create a new project
2. Window > Analysis > UIElements Debugger
3. Search "type, name, or class" in the Search box

Actual Result:
NullReferenceException thrown on searching by "type, name, or class" in the Search Box when no panel is selected in the Panel's dropdown list

Expected Result:
No exception in the console

Note: Search Box of "type, name, or class" in UIElements Debugger window is introduced in 2019.1.a11

Occurring on:
2020.2.0a5, 2020.1.0b4, 2019.3.8f1, 2019.2.13f1, 2019.1.11f1, 2019.1.0a11

Environment:
Windows10 and macOS 10.14

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.UIElements.Debugger.DebuggerTreeView.get_treeItems () (at <89faf899e9c14826b38ffd64a0c1974d>:0)
UnityEditor.UIElements.Debugger.DebuggerSearchBar.PerformSearch (UnityEngine.UIElements.ChangeEvent`1[T] evt) (at <89faf899e9c14826b38ffd64a0c1974d>:0)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventCallbackRegistry.InvokeCallbacks (UnityEngine.UIElements.EventBase evt) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEvent (UnityEngine.UIElements.EventBase evt) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatchUtilities.PropagateEvent (UnityEngine.UIElements.EventBase evt) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.DefaultDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatcher.ApplyDispatchingStrategies (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, System.Boolean imguiEventIsInitiallyUsed) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <6ba4566766d14a388257ed8326fd094d>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <7e072e4699014d9089b4f15a2244620f>:0)