Fix null reference exception thrown if previewing a new file added

In the preview changes dialog we'd call this method to try to get an
icon for it; if the file was a new file we'd throw exceptions which
were swallowed by the shell.

src/VisualStudio/Core/Def/Implementation/Extensions/VisualStudioWorkspaceImplExtensions.cs

@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
@@ -12,6 +10,7 @@
 using Microsoft.VisualStudio.Imaging.Interop;
 using Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem;
 using Microsoft.VisualStudio.Shell.Interop;
+using Roslyn.Utilities;
 
 namespace Microsoft.VisualStudio.LanguageServices.Implementation.Extensions
 {
@@ -73,7 +72,7 @@ public static bool TryGetImageListAndIndex(this VisualStudioWorkspaceImpl worksp
         {
             var hierarchy = workspace.GetHierarchy(id.ProjectId);
             var document = workspace.CurrentSolution.GetDocument(id);
-            if (hierarchy != null)
+            if (hierarchy != null && !RoslynString.IsNullOrEmpty(document?.FilePath))
             {
                 var itemId = hierarchy.TryGetItemId(document.FilePath);
                 return TryGetImageListAndIndex(hierarchy, imageService, itemId, out imageList, out index);