Do not crash the NamingStyleCodeFix when we cannot find the symbol

Fixes #15788

We should always be able to find the symbol that generated this
diagnostic, but this cannot always be done by simply asking for the
declared symbol on the node from the symbol's declaration location.
Issue #16588 is tracking a more complete fix for this that always
successfully finds the symbol.

src/Features/Core/Portable/CodeFixes/NamingStyle/AbstractNamingStyleCodeFixProvider.cs

@@ -36,6 +36,16 @@ public sealed override async Task RegisterCodeFixesAsync(CodeFixContext context)
             var model = await document.GetSemanticModelAsync(context.CancellationToken).ConfigureAwait(false);
             var symbol = model.GetDeclaredSymbol(node, context.CancellationToken);
 
+            // TODO: We should always be able to find the symbol that generated this diagnostic,
+            // but this cannot always be done by simply asking for the declared symbol on the node 
+            // from the symbol's declaration location.
+            // See https://github.com/dotnet/roslyn/issues/16588
+
+            if (symbol == null)
+            {
+                return;
+            }
+
             var fixedNames = style.MakeCompliant(symbol.Name);
             foreach (var fixedName in fixedNames)
             {