When "Disabled" is toggled under the "Options" from the "Search Index Manager" window then it causes the Editor to thrown NullReferenceException, refer attached video.

Steps to repro:
1. Create a new project
2. Window > Package Manager > Quick Search > Install (Preview.7 - 1.6.0)
3. Edit > Preferences > Quick Search
4. Asset indexing mode > Full Indexing > Create Index
5. Click on Index Manager
6. Search index Manger > Assets > Options
7. Toggle Disabled option

Actual Result:
NullReferenceException is thrown when "Disabled" is toggled under "Options" from the "Search Index Manager"

Expected Result:
"Disabled" option is toggled gracefully

Note: Index Manager feature is introduced in package version 1.6.0-preview.7

Occurring on:
2020.2.0a13, 2020.1.0b11 with package version 1.6.0-preview.7, 1.6.0-preview.8

Environment:
Windows 10 and macOS 10.14

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
Unity.QuickSearch.IndexManager.<CreateOptionsVisualElements>b__56_1 (UnityEngine.UIElements.ChangeEvent`1[T] evt) (at Library/PackageCache/com.unity.quicksearch@1.6.0-preview.8/Editor/Indexing/IndexManager.cs:394)
UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventCallbackRegistry.InvokeCallbacks (UnityEngine.UIElements.EventBase evt) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.CallbackEventHandler.HandleEvent (UnityEngine.UIElements.EventBase evt) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatchUtilities.PropagateEvent (UnityEngine.UIElements.EventBase evt) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.DefaultDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.ApplyDispatchingStrategies (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, System.Boolean imguiEventIsInitiallyUsed) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEventQueue () (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.OpenGate () (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcherGate.Dispose () (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <e1c6d781608a4604a07d126a96d78d19>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <781214cdc4654564ac5d6afbf7d06d22>:0)