Opening Serial Monitor Closes IDE (Ubuntu)

I have a Jetson Xavier (ARM 64-bit CPU running Ubuntu 18.04) loaded with the Linux 64-bit Arduino IDE, version 1.8.10. When I try to open the serial monitor from the Tools menu (with my Arduino Uno board plugged in), the IDE suddenly closes - no error message, nothing. Afterwards, any attempt to upload code to the Arduino will get the "device is busy" error until I restart the computer. Does anyone know what might be causing this? Thank you.