Fix `None` `prevKeyPoints` for multi-object tracking (#9386)

Co-authored-by: UltralyticsAssistant <web@ultralytics.com>
Co-authored-by: Laughing <61612323+Laughing-q@users.noreply.github.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/trackers/utils/gmc.py

@@ -319,7 +319,7 @@ class GMC:
         keypoints = cv2.goodFeaturesToTrack(frame, mask=None, **self.feature_params)
 
         # Handle first frame
-        if not self.initializedFirstFrame:
+        if not self.initializedFirstFrame or self.prevKeyPoints is None:
             self.prevFrame = frame.copy()
             self.prevKeyPoints = copy.copy(keypoints)
             self.initializedFirstFrame = True