It occurs error when exectted the final step in demo.py

[STRUGGLE1999]

Traceback (most recent call last): File "demo.py", line 68, in visualize(renderer, original_img, params, vertices, cam, joints) File "../visualise/vis_util.py", line 180, in visualize img_overlay = renderer(vert_shifted, img=img, bg_color=np.array((255.0, 255.0, 255.0, 1))) File "../visualise/trimesh_renderer.py", line 47, in call image_bytes = scene.save_image(resolution=(w, h), background=bg_color, visible=True) File "/root/.local/lib/python3.6/site-packages/trimesh/scene/scene.py", line 869, in save_image scene=self, resolution=resolution, **kwargs) File "/root/.local/lib/python3.6/site-packages/trimesh/viewer/windowed.py", line 900, in render_scene resolution=resolution, **kwargs) File "/root/.local/lib/python3.6/site-packages/trimesh/viewer/windowed.py", line 172, in init caption=caption) File "/root/.local/lib/python3.6/site-packages/pyglet/window/xlib/init.py", line 171, in init super(XlibWindow, self).init(*args, **kwargs) File "/root/.local/lib/python3.6/site-packages/pyglet/window/init.py", line 590, in init display = pyglet.canvas.get_display() File "/root/.local/lib/python3.6/site-packages/pyglet/canvas/init.py", line 94, in get_display return Display() File "/root/.local/lib/python3.6/site-packages/pyglet/canvas/xlib.py", line 123, in init raise NoSuchDisplayException('Cannot connect to "%s"' % name) pyglet.canvas.xlib.NoSuchDisplayException: Cannot connect to "None"

Dose anyone help me ？Dose anyone meet the same problem?