fix: error with code generation when file has unnamed extension

mobx_codegen/lib/src/template/method_override.dart

@@ -43,7 +43,7 @@ class MethodOverrideTemplate {
 
   late String name;
   late String returnType;
-  late SurroundedCommaList<String> returnTypeArgs;
+  late SurroundedCommaList<String?> returnTypeArgs;
 
   late SurroundedCommaList<TypeParamTemplate> _typeParams;
   late SurroundedCommaList<String> _typeArgs;

mobx_codegen/lib/src/type_names.dart

@@ -22,14 +22,14 @@ class LibraryScopedNameFinder {
 
   final LibraryElement library;
 
-  final Map<Element, String> _namesByElement = {};
+  final Map<Element, String?> _namesByElement = {};
 
-  Map<Element, String> get namesByElement {
+  Map<Element, String?> get namesByElement {
     // Add all of this library's type-defining elements to the name map
     final libraryElements =
         library.topLevelElements.whereType<TypeDefiningElement>();
     for (final element in libraryElements) {
-      _namesByElement[element] = element.name!;
+      _namesByElement[element] = element.name;
     }
 
     // Reverse each import's export namespace so we can map elements to their
@@ -58,14 +58,14 @@ class LibraryScopedNameFinder {
   String findReturnTypeName(FunctionTypedElement executable) =>
       _getDartTypeName(executable.returnType);
 
-  List<String> findReturnTypeArgumentTypeNames(ExecutableElement executable) {
+  List<String?> findReturnTypeArgumentTypeNames(ExecutableElement executable) {
     final returnType = executable.returnType;
     return returnType is ParameterizedType
         ? returnType.typeArguments.map(_getDartTypeName).toList()
         : [];
   }
 
-  String findTypeParameterBoundsTypeName(TypeParameterElement typeParameter) {
+  String? findTypeParameterBoundsTypeName(TypeParameterElement typeParameter) {
     assert(typeParameter.bound != null);
     return _getDartTypeName(typeParameter.bound!);
   }
@@ -88,7 +88,6 @@ class LibraryScopedNameFinder {
         typeElement == null ||
             // This is a bare type param, like "T"
             type is TypeParameterType) {
-      // ignore: deprecated_member_use_from_same_package
       return type.getDisplayString();
     }
 

mobx_codegen/test/data/with_extension_input.dart

@@ -0,0 +1,15 @@
+library generator_sample;
+
+import 'package:mobx/mobx.dart';
+
+part 'store_with_extension.g.dart';
+
+// ignore: library_private_types_in_public_api
+class Foo = _Foo with _$Foo;
+
+abstract class _Foo with Store {
+  @observable
+  late String name;
+}
+
+extension on int {}

mobx_codegen/test/mobx_codegen_test.dart

@@ -59,6 +59,11 @@ void main() {
           './data/valid_keep_alive_computed_output.dart');
     });
 
+    test('generates for a class with extension', () async {
+      await compareFiles('./data/with_extension_input.dart',
+          './data/with_extension_output.dart');
+    });
+
     createTests([
       const TestInfo(
           description: 'invalid output is handled',