Generating C# code for a given .proto file produces an error while opening a file

I use google::protobuf::compiler::csharp::Generator to generate .cs file.

First, I create google::protobuf::compiler::Importer. To do so, I need to get an instance of DiskSourceTree and implement MultiFileErrorCollector:

class ErrorCollector : public MultiFileErrorCollector
{
public:
    void AddError(const std::string& filename, int line, int column, const std::string& message) override
    {
        std::fstream stream;
        stream.open(filename);
        stream << message;
        stream.close();
    }
    void AddWarning(const std::string& filename, int line, int column, const std::string& message) override
    {
        std::fstream stream;
        stream.open(filename);
        stream << message;
        stream.close();
    }
};

After that, I implement GeneratorContext to pass it to Generator::Generate():

class Context : public GeneratorContext
{
public:
    google::protobuf::io::ZeroCopyOutputStream* Open(const std::string& filename)
    {
        stream_ptr = std::make_unique<std::fstream>();
        stream_ptr->open(filename);
        proto_stream = std::make_unique<google::protobuf::io::OstreamOutputStream>(stream_ptr.get());
        return proto_stream.get();
    }

private:
    std::unique_ptr<std::fstream> stream_ptr;
    std::unique_ptr<google::protobuf::io::OstreamOutputStream> proto_stream;
};

The error occurs at the stage of importing .proto-file. The debugger says, const google::protobuf::FileDescriptor* desc = importer->Import(FILENAME); results to null.

It's probably something with the file path or even with my understanding of how it all works. I would appreciate any help.

Here's my main function:

int main()
{
        // building an Importer
        DiskSourceTree* tree = new DiskSourceTree;
        ErrorCollector* collector = new ErrorCollector;

        Importer* importer = new Importer(tree, collector);
    
        const std::string FILENAME = "C:/my/path/my_file.proto";
        const google::protobuf::FileDescriptor* desc = importer->Import(FILENAME); // the error is here
    
        // generating the code
        Generator generator;
        Context* context = new Context();
        std::string* error_str = new std::string;
        error_str->reserve(256);
    
        if (generator.Generate(desc, "", context, error_str)) // this line produces an exception since the descriptor is invalid
        {
            std::cout << "success!";
        }
    
        delete tree;
        delete collector;
        delete importer;
        delete context;
        delete error_str;
    }