Crash While Using AI Polygons

[Erotemic]

Provide environment information

Same env as: #1334

What OS are you using?

Ubuntu 22.04

Describe the Bug

Output I get is:

I was creating a lot of postive and negative anchor points fairly quickly when this crash occurred

MD5 matches: /home/joncrall/.cache/gdown/https-COLON--SLASH--SLASH-github.com-SLASH-wkentaro-SLASH-labelme-SLASH-releases-SLASH-download-SLASH-sam-20230416-SLASH-sam_vit_h_4b8939.quantized.decoder.onnx
Traceback (most recent call last):
  File "/home/joncrall/code/labelme/labelme/widgets/canvas.py", line 773, in paintEvent
    points = self._ai_model.predict_polygon_from_points(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/joncrall/code/labelme/labelme/ai/models/segment_anything.py", line 64, in predict_polygon_from_points
    polygon = _compute_polygon_from_points(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/joncrall/code/labelme/labelme/ai/models/segment_anything.py", line 172, in _compute_polygon_from_points
    contour = max(contours, key=_get_contour_length)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: max() arg is an empty sequence
Aborted (core dumped)

It looks like there should be some error handling when countours is empty here.

Expected Behavior

Not crashing.

To Reproduce

I wasn't able to reproduce with random clicks. There must have been something peculiar about the particular shape I made when the crash happened. If I can determine the shape that causes it, I will post back with more details. Either way, from the error we can determine that the assumption that the countours list is non-empty does not hold, and error handling should be added to the code at that point.