Proper way to stop a video recording (finishWriting) with AVAssetWriter and without crashing

Question

I record videos with AVAssetWriter. The user can send the video, then I call finishWriting, or cancel the recording, then I call cancelWriting.

How I record:

func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, from connection: AVCaptureConnection!){
        guard !message_to_send else{
            return
        }
        guard is_recording else{
            return
        }
        guard CMSampleBufferDataIsReady(sampleBuffer) else{
            print("data not ready")
            return
        }
        guard let w=file_writer else{
            print("video writer nil")
            return
        }
        guard let sb=sampleBuffer else{
            return
        }

        if w.status == .unknown /*&& start_recording_time==nil*/{
            if captureOutput==video_output{
                print("\nSTART RECORDING")
                w.startWriting()
                start_recording_time=CMSampleBufferGetPresentationTimeStamp(sb)
                w.startSession(atSourceTime: start_recording_time!)
            }else{
                return
            }
        }

        if w.status == .failed{
            print("failed with error:", w.error ?? "")
            stop_record()
            return
        }

        guard w.status == .writing else{
            print("not writing")
            return
        }

        if captureOutput==audio_output{
            if audio_writer.isReadyForMoreMediaData && video_written{
                audio_writer.append(sb)
            }else{
                print("audio writer not ready OR video not written yet")
            }
        }else if captureOutput==video_output{
            if video_writer.isReadyForMoreMediaData /*&& is_recording*/{
                video_writer.append(sb)   // << this line gets called after stop_record is called and causes a crash
                if !video_written{
                    print("added 1st video frame")
                    video_written=true
                }
                //print("write video")
            }else{
                print("video writer not ready")
            }
        }
    }
}

How I stop the recording:

func stop_record(){
        print("\nstop_record")
        guard is_recording else{
            print("not recording > return")
            return
        }

        is_recording=false
        cam_q.async{
            self.is_recording=false
        }

        print("file writer status", file_writer.status.rawValue)
        if file_writer.status.rawValue==1{
            self.video_writer?.markAsFinished()
            print("video finished")
            self.audio_writer?.markAsFinished()
            print("audio finished")
        }else{
            print("not writing")
        }

        if !self.message_to_send{
            print("cancel writing")
            self.file_writer.cancelWriting()
            return
        }

        file_writer.finishWriting(){
            print("finished writing")
            DispatchQueue.main.async{
                let st=self.file_writer.status
                if st == .failed{
                    print("status: failed")
                }else if st == .completed{
                    print("status: completed")
                }else if st == .cancelled{
                    print("status: cancelled")
                }else{
                    print("status: unknown")
                }

                if let e=self.file_writer.error{
                    print("stop record error:", e)
                }

                if self.message_to_send{
                    send_message()
                }
            }
        }
    }

When I call stop_record, sometimes it crashes. Even if file_writer is supposed to stop writing and if I make several verifications in didOutputSampleBuffer, this line gets called:

video_writer.append(sb)

And I get:

stop_record

file writer status 1

video finished

audio finished

libc++abi.dylib: terminating with uncaught exception of type NSException

I tried the same code without calling markAsFinished but it also crashes (more often I'd say).

I'm wondering if this is a thread issue (since stop_record and didOutputSampleBuffer methods are not called on the same thread). Anyway, I don't know what I'm doing wrong. If someone could help me on this, I'd really appreciate.