Make AtDirectiveCompletionPoint resilient to null owners.

- Added a test to verify the new owner == null case.

#1733

src/Microsoft.VisualStudio.LanguageServices.Razor/Editor/RazorDirectiveCompletionProvider.cs

@@ -147,12 +147,19 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor.Editor
             return completionItems;
         }
 
-        private bool AtDirectiveCompletionPoint(RazorSyntaxTree syntaxTree, CompletionContext context)
+        // Internal for testing
+        internal bool AtDirectiveCompletionPoint(RazorSyntaxTree syntaxTree, CompletionContext context)
         {
             if (TryGetRazorSnapshotPoint(context, out var razorSnapshotPoint))
             {
                 var change = new SourceChange(razorSnapshotPoint.Position, 0, string.Empty);
                 var owner = syntaxTree.Root.LocateOwner(change);
+
+                if (owner == null)
+                {
+                    return false;
+                }
+
                 if (owner.ChunkGenerator is ExpressionChunkGenerator &&
                     owner.Symbols.All(IsDirectiveCompletableSymbol) &&
                     // Do not provide IntelliSense for explicit expressions. Explicit expressions will usually look like:

test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Editor/RazorDirectiveCompletionProviderTest.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.ComponentModel.Composition;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -29,6 +30,24 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor.Editor
             CSharpCodeParser.TagHelperPrefixDirectiveDescriptor,
         };
 
+        [Fact]
+        public void AtDirectiveCompletionPoint_ReturnsFalseIfChangeHasNoOwner()
+        {
+            // Arrange
+            var codeDocumentProvider = CreateCodeDocumentProvider("@", Enumerable.Empty<DirectiveDescriptor>());
+            var completionProvider = new FailOnGetCompletionsProvider(codeDocumentProvider);
+            var document = CreateDocument();
+            codeDocumentProvider.Value.TryGetFromDocument(document, out var codeDocument);
+            var syntaxTree = codeDocument.GetSyntaxTree();
+            var completionContext = CreateContext(2, completionProvider, document);
+
+            // Act
+            var result = completionProvider.AtDirectiveCompletionPoint(syntaxTree, completionContext);
+
+            // Assert
+            Assert.False(result);
+        }
+
         [Fact]
         public async Task GetDescriptionAsync_AddsDirectiveDescriptionIfPropertyExists()
         {