Added warning when app is already open

main.py

@@ -90,13 +90,12 @@ def except_hook(cls, exception, traceback):
     std_err_handler.err_msg.connect(tray_icon.std_err_post)
 
     # Errors that occur in the init phase aren't caught by the stderr.
-    tray_icon.show()
-    tray_icon.start_app()
-    # try:
-    #
-    # except Exception as e:
-    #     QMessageBox.critical(None, "Error during start-up", f"An error occurred during start-up:\n\n{traceback.format_exc()}")
-    #     logger.critical("Uncaught exception", exc_info=(type(e).__class__, e, e.__traceback__))
+    try:
+        tray_icon.show()
+        tray_icon.start_app()
+    except Exception as e:
+        QMessageBox.critical(None, "Error during start-up", f"An error occurred during start-up:\n\n{traceback.format_exc()}")
+        logger.critical("Uncaught exception", exc_info=(type(e).__class__, e, e.__traceback__))
 
     app.exec()
     print("This should not be printed.")

tray_icon.py

@@ -26,6 +26,7 @@ def __init__(self, icon, parent=None):
         self.setContextMenu(menu)
 
         self.activated.connect(self.onClick)
+
         self.thread = VolumeThread()
 
     def std_err_post(self, msg):

volume_thread.py

@@ -1,13 +1,17 @@
+import sys
 from pathlib import Path
 
 import serial
 from PyQt5.QtCore import QThread
 import logging
 
+from PyQt5.QtWidgets import QMessageBox
+
 from control import Control
 
 logger = logging.getLogger('root')
 
+
 class VolumeThread(QThread):
 
     def __init__(self):
@@ -16,7 +20,16 @@ def __init__(self):
         self.running = True
         self.control = Control(Path.cwd() / 'mapping.txt')
         logger.info("Setting up serial communication.")
-        self.arduino = serial.Serial(self.control.port, self.control.baudrate, timeout=.1)
+        try:
+            self.arduino = serial.Serial(self.control.port, self.control.baudrate, timeout=.1)
+        except serial.SerialException as e:
+            msg = QMessageBox.critical(None, "Application already running",
+                                       "The application crashed because the serial "
+                                       "connection is busy. This may mean that another "
+                                       "instance is already running. Please check the "
+                                       "system tray or the task manager.")
+
+            raise
 
     def run(self):
         logger.info("Entering thread loop.")
@@ -26,4 +39,4 @@ def run(self):
                 data = str(self.arduino.readline()[:-2], 'utf-8')  # Trim off '\r\n'.
                 if data:
                     values = [float(val) for val in data.split('|')]
-                    self.control.set_volume(values)
+                    self.control.set_volume(values)