Apply suggestions from code review

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: cynthiajoan

packages/firebase_ai/firebase_ai/example/lib/pages/function_calling_page.dart

@@ -91,9 +91,13 @@ class _FunctionCallingPageState extends State<FunctionCallingPage> {
         ),
       },
       callable: (args) async {
-        final cuisine = args['cuisine']! as String;
-        final location = args['location']! as String;
-        return findRestaurants(cuisine, location);
+        final cuisine = args['cuisine'];
+        final location = args['location'];
+        if (cuisine is String && location is String) {
+          return findRestaurants(cuisine, location);
+        }
+        // It's good practice to handle cases where arguments are missing or have the wrong type.
+        throw Exception('Missing or invalid arguments for findRestaurants');
       },
     );
     _autoGetRestaurantMenuTool = AutoFunctionDeclaration(

packages/firebase_ai/firebase_ai/lib/src/chat.dart

@@ -39,7 +39,7 @@ final class ChatSession {
       : _autoFunctionDeclarations = _tools
             ?.expand((tool) => tool.autoFunctionDeclarations)
             .fold(<String, AutoFunctionDeclaration>{}, (map, function) {
-          map?[function.name] = function;
+          map[function.name] = function;
           return map;
         });
   final Future<GenerateContentResponse> Function(Iterable<Content> content,