fix: handle case when sub device is not found in DeviceServiceImpl

sample/src/main/java/com/dji/sample/manage/service/impl/DeviceServiceImpl.java

@@ -280,7 +280,13 @@ public class DeviceServiceImpl extends ServiceImpl<IDeviceMapper, DeviceEntity>
             return;
         }
 
-        DeviceDTO subDevice = getDevicesByParams(DeviceQueryParam.builder().deviceSn(gateway.getChildDeviceSn()).build()).get(0);
+        List<DeviceDTO> subDevices = getDevicesByParams(DeviceQueryParam.builder().deviceSn(gateway.getChildDeviceSn()).build());
+        if (CollectionUtils.isEmpty(subDevices)) {
+            log.warn("sub device not found, gatewaySn={}, childDeviceSn={}", gateway.getDeviceSn(), gateway.getChildDeviceSn());
+            return;
+        }
+
+        DeviceDTO subDevice = subDevices.get(0);
         subDevice.setStatus(deviceRedisService.checkDeviceOnline(subDevice.getDeviceSn()));
         gateway.setChildren(subDevice);