"Bootloader install failed" recovery gets stuck: Pressing OK does nothing

[Scott]

The "Bootloader install failed" dialog that is presented during the installation process when bootloader installation fails doesn't work: Pressing OK has no effect. The OK button UI element darkens to indicate that it was clicked, but nothing happens -- the window does not go away, no additional errors are presented, and the bootloader is not installed at the requested location. Selecting the "Continue without a bootloader" or "Cancel the installation" options and pressing OK likewise has no effect.

Things seem thoroughly hung in this state: Clicking on the "close" red circle with white X in the upper left has no effect. Even the log-out and power-off options in the top-left-corner-of-the-screen menu do nothing. Pressing cntr-alt-F2 to switch to another virtual console and pressing ctrl-alt-delete to restart the machine *does* work.

To reproduce:

On another Ubuntu system:
1. Download ubuntu-18.10-desktop-amd64.iso.
2. Use Startup Disk Creator (usb-creator-common/bionic,now 0.3.5 amd64) to load the installation ISO onto the SD card.
3. Write-lock the SD card using the slide-switch on the side.

On the empty computer onto which Ubuntu will be installed:
4. Boot from the SD card.
5. Select "Install Ubuntu".
6. Select "Normal installation".
7. Select automatic partitioning with encryption: "Erase disk and install Ubuntu" + "Encrypt the new Ubuntu installation for security".
8. Respond to key, timezone, username, hostname, etc. prompts. Installation churns for awhile, saying "Copying files...", "Installing system", "Configuring hardware...", etc.

(At this point, the installation goes awry because automatic partitioning chose to install the bootloader to the SD card (mmcblk0) instead of the SSD (sda) where it should go, but that is a separate problem tracked in bug 1809163. This bug tracks the failure of the "Bootloader install failed" dialog to allow recovery from this situation.)

9. Installation says "Running 'grub-install /dev/mmcblk0'..."
10. An error box comes up:
     Unable to install GRUB in /dev/mmcblk0
     Executing 'grub-install /dev/mmcblk0' failed.
     This is a fatal error.
     [OK]
11. After pressing OK, this dialog appears:
    Bootloader install failed
    (X) Sorry, an error occurred and it was not possible to install the bootloader at the specified location.
    How would you like to proceed?
      (o) Choose a different device to install the bootloader on:
        [/dev/mmcblk0]
      ( ) Continue without a bootloader.
      ( ) Cancel the installation.
    [OK]

The device selection drop-down has eight choices (including two apparent duplicates):
  /dev/mapper/sda5_crypt Linux device-mapper (crypt) (255.3 GB)
  /dev/mapper/sda5_crypt
  /dev/mapper/ubuntu--vg-root Linux device-mapper (linear) (254.3 GB)
  /dev/mapper/ubuntu--vg-root
  /dev/mapper/ubuntu--vg-swap_1
  /dev/sda ATA SAMSUNG SSD XXXX (256.1 GB)
  /dev/sda1
  /dev/sda5

This is the screen that is stuck as described above.