Make sure we don't attempt to analyze methods without a symbol

Since we need a resolved invocation syntax to an actual symbol in order to determine if it's the service collection method we are looking for.

Author: kzu

src/DependencyInjection/AddServicesAnalyzer.cs

@@ -59,7 +59,7 @@ static bool IsDDICode(SyntaxNode node, SemanticModel semantic)
                     .OfType<InvocationExpressionSyntax>()
                     .Select(invocation => new { Invocation = invocation, semantic.GetSymbolInfo(invocation, semanticContext.CancellationToken).Symbol })
                     // It has to be user-provided code, not our own extensions/overloads.
-                    .Where(x => !IsDDICode(x.Invocation, semantic))
+                    .Where(x => !IsDDICode(x.Invocation, semantic) && x.Symbol != null)
                     .Select(x => new { x.Invocation, Method = (IMethodSymbol)x.Symbol! });
 
                 bool IsServiceCollectionExtension(IMethodSymbol method) => method.IsExtensionMethod &&