releaser-upgrade failure 16.04.6LTS

[Russ Herman]

Trying to get to 18.04
Stripped out all non-standard PPA references and reran update/upgrade, completed ok.
But do-release-upgrade failed with with

(appstreamcli:8886): GLib-CRITICAL **: g_strchug: assertion 'string != NULL' failed

(appstreamcli:8886): GLib-CRITICAL **: g_strchomp: assertion 'string != NULL' failed

(appstreamcli:8886): GLib-CRITICAL **: g_strchug: assertion 'string != NULL' failed

(appstreamcli:8886): GLib-CRITICAL **: g_strchomp: assertion 'string != NULL' failed

(appstreamcli:8886): GLib-CRITICAL **: g_strchug: assertion 'string != NULL' failed

(appstreamcli:8886): GLib-CRITICAL **: g_strchomp: assertion 'string != NULL' failed
AppStream cache update completed, but some metadata was ignored due to errors.

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done

Calculating the changes

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.4.0-146.172-generic 4.4.177
Uname: Linux 4.4.0-146-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
Date: Thu Apr 11 00:08:23 2019
InstallationDate: Installed on 2012-07-06 (2469 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-04-11 (0 days ago)
mtime.conffile..etc.update-manager.release-upgrades: 2019-04-10T22:40:31.357962

[Ubuntu Foundations Team Bug Bot (crichton)]

Reviewing the files from your upgrade attempt you seem to have enabled a PPA that provides Xorg packages. This will prevent the upgrade as that PPA contains different versions of packages essential to Ubuntu. The page, https://launchpad.net/~xorg-edgers/+archive/ppa, contains details on to how to revert to the official Xorg packages for the xorg-edgers PPA. Instructions for reverting to official Xorg packages should be similar for the PPA that you have enabled. Thanks.

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[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug 1069133, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Please continue to report any other bugs you may find.
[This is an automated message. I apologize if it reached you inappropriately; please just reply to this message indicating so.]