fix(cameras): snapshot stop_event in read loops to avoid None deref

The background read loops accessed self.stop_event repeatedly while
_stop_read_thread() can reassign it to None after join(). Reading the
attribute across the loop condition (and a mid-loop re-check) was a
time-of-check/time-of-use race: stop_event could flip to None between
the `is None` test and the `.is_set()` call, raising AttributeError on
the worker thread.
Snapshot self.stop_event into a local once, guard it, and loop on the
local Event. The Event object is thread-safe and lives for the thread's
lifetime; _stop_read_thread() always calls .set() before nulling the
attribute, so the local observes the stop and exits cleanly. This also
lets us drop the redundant pre-lock stop check.
Applies to OpenCVCamera, RealSenseCamera, and ZMQ camera.

src/lerobot/cameras/opencv/camera_opencv.py

@@ -442,11 +442,12 @@ class OpenCVCamera(Camera):
 
         Stops on DeviceNotConnectedError, logs other errors and continues.
         """
-        if self.stop_event is None:
+        stop_event = self.stop_event
+        if stop_event is None:
             raise RuntimeError(f"{self}: stop_event is not initialized before starting read loop.")
 
         failure_count = 0
-        while not self.stop_event.is_set():
+        while not stop_event.is_set():
             try:
                 raw_frame = self._read_from_hardware()
                 processed_frame = self._postprocess_image(raw_frame)
@@ -486,6 +487,7 @@ class OpenCVCamera(Camera):
             self.thread.join(timeout=2.0)
 
         self.thread = None
+        self.stop_event = None
 
         with self.frame_lock:
             self.latest_frame = None

src/lerobot/cameras/realsense/camera_realsense.py

@@ -471,11 +471,12 @@ class RealSenseCamera(Camera):
 
         Stops on DeviceNotConnectedError, logs other errors and continues.
         """
-        if self.stop_event is None:
+        stop_event = self.stop_event
+        if stop_event is None:
             raise RuntimeError(f"{self}: stop_event is not initialized before starting read loop.")
 
         failure_count = 0
-        while not self.stop_event.is_set():
+        while not stop_event.is_set():
             try:
                 frame = self._read_from_hardware()
                 color_frame_raw = frame.get_color_frame()

src/lerobot/cameras/zmq/camera_zmq.py

@@ -246,11 +246,12 @@ class ZMQCamera(Camera):
         """
         Internal loop run by the background thread for asynchronous reading.
         """
-        if self.stop_event is None:
+        stop_event = self.stop_event
+        if stop_event is None:
             raise RuntimeError(f"{self}: stop_event is not initialized.")
 
         failure_count = 0
-        while not self.stop_event.is_set():
+        while not stop_event.is_set():
             try:
                 frame = self._read_from_hardware()
                 capture_time = time.perf_counter()