Don't attempt to provide a navbar if there is no navbar language service.

src/VisualStudio/Core/Def/Implementation/LanguageService/AbstractLanguageService`2.VsCodeWindowManager.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System.Diagnostics;
+using System.Linq;
 using Microsoft.CodeAnalysis.Editor;
 using Microsoft.CodeAnalysis.Editor.Options;
 using Microsoft.CodeAnalysis.Options;
+using Microsoft.CodeAnalysis.Shared.Extensions;
+using Microsoft.CodeAnalysis.Text;
 using Microsoft.VisualStudio.LanguageServices.Implementation.NavigationBar;
 using Microsoft.VisualStudio.TextManager.Interop;
 using Roslyn.Utilities;
@@ -62,6 +65,19 @@ private void AddOrRemoveDropdown(bool enabled)
                     return;
                 }
 
+                if (ErrorHandler.Failed(_codeWindow.GetBuffer(out var buffer)))
+                {
+                    return;
+                }
+
+                // Temporary solution until the editor provides a proper way to resolve the correct navbar.
+                // Tracked in https://github.com/dotnet/roslyn/issues/40989
+                var document = _languageService.EditorAdaptersFactoryService.GetDataBuffer(buffer).AsTextContainer().GetRelatedDocuments().FirstOrDefault();
+                if (document.GetLanguageService<INavigationBarItemService>() == null)
+                {
+                    return;
+                }
+
                 if (enabled)
                 {
                     var existingDropdownBar = GetDropdownBar(dropdownManager);