AttributeError: 'NoneType' object has no attribute 'serialize' Error in converting .onnx model to .trt

I am using this project in Xavier NX. According to their instructions I am running the demo with.

python3 ctdet --demo webcam --arch hardnet_85 --load_model ./../models/centernet_hardnet85_coco.pth --input_w 416 --input_h 416
It first converts model to .onnx and then .onnx to .trt. It have converted to .onnx successfully but have this error during .onnx to .trt conversion.

I have tried pytorch 1.6.0 and pytorch 1.5.0 , what is the main reason? is it TensorRT problem or it is pytorch problem ? Please see my attached error and systems specs screenshots.