Error when selecting "isolinux" on the Boot tab while customizing Xubuntu.

The following error occurs while customizing xubuntu-20.10-desktop-amd64.iso.

1. Navigate to the Options page, Boot tab.
2. Click on "Show all files" toggle button in the headerbar.
3. Scroll down and click on "isolinux".

      • Change tree selection to............... boot/grub
      • Show pane for file..................... boot/grub
      • Change tree selection to............... isolinux
    Traceback (most recent call last):
      File "/usr/share/cubic/cubic/utilities/files_tree.py", line 361, in change_tree_selection
        mime_type = file_utilities.read_mime_type(full_file_path)
      File "/usr/share/cubic/cubic/utilities/file_utilities.py", line 434, in read_mime_type
        mime_info = magic.from_file(full_file_path, True)
      File "/usr/lib/python3/dist-packages/magic/__init__.py", line 180, in from_file
        return m.from_file(filename)
      File "/usr/lib/python3/dist-packages/magic/__init__.py", line 113, in from_file
        with _real_open(filename):
    FileNotFoundError: [Errno 2] No such file or directory: '/mnt/ram/Ubuntu/custom-disk/isolinux'

While navigating from Packages page to the Options page on Next action, the following warning is printed to the log:

      Search and replace in file
      • File path.............................. isolinux/txt.cfg
      • Waring................................. No tree iter found

Furthermore, items can not be added to the "isolinux" directory.

1. Select the "isolinux" directory.
2. Click on the "Create a new file" toggle button in the headerbar.
3. Create a new file named "test".

      • Show pane for create file.............. isolinux
      • Create file............................ isolinux/test
    Traceback (most recent call last):
      File "/usr/share/cubic/cubic/utilities/files_tab.py", line 383, in create_file
        with open(full_file_path, 'a'):
    FileNotFoundError: [Errno 2] No such file or directory: '/mnt/ram/Ubuntu/custom-disk/isolinux/test'