Better error handling in highlight references

If we could not get the symbol to HR from the semantic model, do not try
creating tags for it. This fixes the HR crash part of #4918

src/EditorFeatures/Core/Implementation/ReferenceHighlighting/AbstractDocumentHighlightsService.cs

@@ -29,18 +29,22 @@ public async Task<IEnumerable<DocumentHighlights>> GetDocumentHighlightsAsync(Do
             }
 
             symbol = await GetSymbolToSearchAsync(document, position, semanticModel, symbol, cancellationToken).ConfigureAwait(false);
+            if (symbol == null)
+            {
+                return SpecializedCollections.EmptyEnumerable<DocumentHighlights>();
+            }
 
             // Get unique tags for referenced symbols
             return await GetTagsForReferencedSymbolAsync(symbol, ImmutableHashSet.CreateRange(documentsToSearch), solution, cancellationToken).ConfigureAwait(false);
         }
 
-        private async Task<ISymbol> GetSymbolToSearchAsync(Document document, int position, SemanticModel semanticModel, ISymbol symbols, CancellationToken cancellationToken)
+        private async Task<ISymbol> GetSymbolToSearchAsync(Document document, int position, SemanticModel semanticModel, ISymbol symbol, CancellationToken cancellationToken)
         {
-            // see whether we can use symbols as it is
+            // see whether we can use the symbol as it is
             var currentSemanticModel = await document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
             if (currentSemanticModel == semanticModel)
             {
-                return symbols;
+                return symbol;
             }
 
             // get symbols from current document again