text: Title: BUG: GUI silently crashes if `classes.txt` is not found
  Body: When a folder is opened in `labelImg` GUI that doesn't have `classes.txt`, the GUI silently crashes without showing any error popup.
  
  ### Steps to Reproduce
  - Put some images and corresponding annotation text files in a test folder.
  - DON'T create `classes.txt`.
  - Start `labelImg` GUI, and open the test folder using *Open Directory*.
  - `labelImg` tries to read `classes.txt` in the test folder, and prints `FileNotFound` error to the console.
  - **No error popup is shown in the GUI** and the program crashes after a few moments.
  
  ### Environment
  - **OS:** Windows 11
  - **PyQt version:** 5.5.19
  - **Python version:** 3.11
  