run demo.py error in branch py36

[azuredsky]

Hi, when run python demo.py in branch py36 :

Traceback (most recent call last):
File "/work/GIT/07_Track/SPLT/demo.py", line 782, in
tracker = MobileTracker(vot=use_vot, dis=display)
File "/work/GIT/07_Track/SPLT/demo.py", line 101, in init
= build_box_predictor(model, model_scope, self.initConstantOp, reuse=None)
File "/work/GIT/07_Track/SPLT/utils/tracking_utils.py", line 466, in build_box_predictor
prediction_dict = model.predict_box_with_init(init_feature_maps, preprocessed_images, istraining=False)
File "/work/GIT/07_Track/SPLT/core/man_meta_arch.py", line 399, in predict_box_with_init
self._anchors = self._anchor_generator.generate(feature_map_spatial_dims)
File "/work/GIT/07_Track/SPLT/lib/object_detection/core/anchor_generator.py", line 101, in generate
anchors = self._generate(feature_map_shape_list, **params)
File "/work/GIT/07_Track/SPLT/lib/object_detection/anchor_generators/multiple_grid_anchor_generator.py", line 202, in _generate
concatenated_anchors = box_list_ops.concatenate(anchor_grid_list)
File "/work/GIT/07_Track/SPLT/lib/object_detection/core/box_list_ops.py", line 529, in concatenate
raise ValueError('all elements of boxlists should be BoxList objects')
ValueError: all elements of boxlists should be BoxList objects