Cubic hangs when customizing a manually created ISO.

Note:
This Question & Answer were inspired by Bug #1821263.
I am posting it here in case it helps someone with a similar problem).

SOLUTION
-------------
The structure of your manually customized ISO may not be correct.

Fortunately, Cubic has a feature to repair this.

WHAT YOU NEED
-------------

(1)
The latest version of Cubic.

    $ sudo apt update
    $ sudo apt install cubic
    $ dpkg -l cubic

(2)
Download an official *.iso from the Ubuntu website. Since you mentioned that you had customized 18.04 you may want to get http://releases.ubuntu.com/18.04/ubuntu-18.04.2-desktop-amd64.iso. (The exact version doesn't matter, so it's a good idea to get the most recent "point" release for your Ubuntu version).

(3)
Copies of all of your custom preseed files (ex. preseed.cfg) and all of your custom boot configuration files (ex. grub.cfg).

BACKUP
------
Just in case something goes wrong, make a backup of your original files.

    $ cp -r /isotest6 /isotest6.backup

Be sure to follow the next steps using the original directory ("/isotest6"), not the backup directory ("/isotest6.backup").

STEPS
-----

(1)
Launch Cubic.

On the Project Directory page, select your project directory (ex. /isotest6).
Click Next.

(2)
You will see the Existing Project page.
Make sure the "Create a disk image from the existing project" radio button is selected.
In the original ISO section, in the Filename field, you will see your original ISO (ex. /isofile/live-cd.iso).
Click the Select button next to this file.
In the File Chooser dialog, pick the official *.iso you had downloaded earlier (ex. ubuntu-18.04.2-desktop-amd64.iso) and click Select.
Don't worry, Cubic knows you had made customizations (ex. /isofile/live-cd.iso), and these will be retained.
This step will allow Cubic to "rebuild" the correct file structure based on an official *.iso file.
Click Next.

You will briefly see a progress bar with the heading "Copy ISO files from the new original disk image."

(3)
Then you will see the Options page.

Click the Package Manifest tab.
Select every package that should be removed for a typical install.
(Note that packages removed for a typical install are also automatically removed for a minimal install).
Also, select every additional package that should be removed for a minimal install.

Click the ISO Boot Kernel tab.
The kernel that you had used for your previous ISO image (ex. live-cd.iso), may not be listed, but that's OK.
Select the radio button for the kernel that should be used to bootstrap your new customized ISO. The suggested default is fine to use.

Click the Preseeed Files tab.
Create new preseed files that are not listed.
Delete any preseed files that you do not need.
For each preseed file, copy (CTRL-C) and paste (CTRL-V) the contents from your original preseed files that you had made copies of.

Click the ISO Boot Configurations tab.
For each boot configuration file, copy (CTRL-C) and paste (CTRL-V) the contents from your original boot configuration files that you had made copies of.
You should be able to leave unused boot configuration files blank, if you don't need them (ex. loopback.cfg, txt.cfg).

(4)
Click the Generate button.

Note: Any changes you make on this page are only saved wen you click the Generate button. If you click the Back or Quit buttons, your changes will be lost.

(5)
You will see the Repackage ISO page.

Cubic will generate the your new customized ISO.

NEXT STEPS
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(a) If you want to keep customizing this ISO, you can continue do it using this project. The Original ISO section on the Existing Project page will continue to say "ubuntu-18.04.2-desktop-amd64.iso", but there is no need to change this in the future.

(b) Alternatively, you can also create a brand new project in Cubic. In this case, simply select your newly created ISO to begin customizing it. The Original ISO section on the Existing Project page will continue to show the name of your newly created ISO.