修复BaseType为空的bug

ModeTypeGenerator.cs

@@ -46,7 +46,7 @@ namespace FlutterCodeGenerator
                 {
                     return new BoolNullableModeType(argName);
                 }
-                else if (type == typeof(string) || type == typeof(char) || type.BaseType.Name == "BaseParamsString" || type.BaseType.Name == "BaseString")
+                else if (type == typeof(string) || type == typeof(char) || type.BaseType?.Name == "BaseParamsString" || type.BaseType?.Name == "BaseString")
                 {
                     return new StringModeType(argName);
                 }