[Bug 1768561] Re: Update to Ubuntu 18.04 LTS fails

[Karun]

how to 
-downgrade ppa's packages to genuine version
- un-pin 'unity' (The package 'unity' is marked for removal but it is in the removal blacklist)
-deactivate ppas
?
i saw one ppa from webupdjava8 that i removed using ppa-purge. still nothing.
same error.
apt-get update fails with 

(appstreamcli:5778): GLib-CRITICAL **: g_strchug: assertion 'string !=
NULL' failed

(appstreamcli:5778): GLib-CRITICAL **: g_strchomp: assertion 'string !=
NULL' failed

(appstreamcli:5778): GLib-CRITICAL **: g_strchug: assertion 'string !=
NULL' failed

(appstreamcli:5778): GLib-CRITICAL **: g_strchomp: assertion 'string !=
NULL' failed

(appstreamcli:5778): GLib-CRITICAL **: g_strchug: assertion 'string !=
NULL' failed

(appstreamcli:5778): GLib-CRITICAL **: g_strchomp: assertion 'string !=
NULL' failed

how to fix?

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https://bugs.launchpad.net/bugs/1768561

Title:
  Update to Ubuntu 18.04 LTS fails

Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Released

Bug description:
  Description:	Ubuntu 16.04.4 LTS
  Release:	16.04

  The update crashes with the following message:
  "Calculating the changes

  Could not calculate the upgrade

  An unresolvable problem occurred while calculating the upgrade.

  This can be caused by: 
  * Upgrading to a pre-release version of Ubuntu 
  * Running the current pre-release version of Ubuntu 
  * Unofficial software packages not provided by Ubuntu 

  If none of this applies, then please report this bug using the 
  command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. 

  
  Restoring original system state"

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: ubuntu-release-upgrader-core 1:16.04.25
  ProcVersionSignature: Ubuntu 4.13.0-39.44~16.04.1-generic 4.13.16
  Uname: Linux 4.13.0-39-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.16
  Architecture: amd64
  CrashDB: ubuntu
  CurrentDesktop: Unity
  Date: Wed May  2 20:14:30 2018
  InstallationDate: Installed on 2018-01-31 (90 days ago)
  InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
  PackageArchitecture: all
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to xenial on 2018-05-02 (0 days ago)

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