AttributeError: 'DistUpgradeController' object has no attribute 'tasks'

[amichair]

While upgrading Kubuntu 18.04 to 20.04.1, the upgrader tool crashed with the stack trace below.
I then ran apt update && apt full-upgrade manually from terminal, restarted into a broken login screen, found out plasma is not installed, and manually installed all packages in the distribution manifest to fix the system. It all started with this:

Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-snwfpw49/focal", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeMain.py", line 238, in main
    if app.run():
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeController.py", line 2089, in run
    return self.fullUpgrade()
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeController.py", line 2048, in fullUpgrade
    if not self.calcDistUpgrade():
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeController.py", line 1118, in calcDistUpgrade
    if not self.cache.installTasks(self.tasks):
AttributeError: 'DistUpgradeController' object has no attribute 'tasks'
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/problem_report.py", line 497, in add_to_existing
    self.write(f)
  File "/usr/lib/python3/dist-packages/problem_report.py", line 450, in write
    block = f.read(1048576)
  File "/usr/lib/python3.6/codecs.py", line 321, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte

Original exception was:
Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-snwfpw49/focal", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeMain.py", line 238, in main
    if app.run():
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeController.py", line 2089, in run
    return self.fullUpgrade()
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeController.py", line 2048, in fullUpgrade
    if not self.calcDistUpgrade():
  File "/tmp/ubuntu-release-upgrader-snwfpw49/DistUpgrade/DistUpgradeController.py", line 1118, in calcDistUpgrade
    if not self.cache.installTasks(self.tasks):
AttributeError: 'DistUpgradeController' object has no attribute 'tasks'