autoremoval of kernels breaks unless other packages need updating

[arQon]

after updating multiple packages including the kernel, choose "restart later".
note: in this particular case i deferred the update for a specific package (thunderbird) - no idea if that's required to trigger this bug or not.

when update manager pops up again after about 10s (which is technically "later", i suppose, but pointlessly short) it will then correctly show the grandfather kernel as "unused" for autoremoval - but when the checkbox for the unwanted thunderbird update is cleared, the "install now" button greys out since there are then no updates to install, which blocks the removal of the now-obsolete kernel.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: update-manager 1:16.04.15
ProcVersionSignature: Ubuntu 4.4.0-138.164-generic 4.4.155
Uname: Linux 4.4.0-138-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: MATE
Date: Fri Nov 16 12:57:36 2018
EcryptfsInUse: Yes
GsettingsChanges:
 b'com.ubuntu.update-manager' b'window-height' b'893'
 b'com.ubuntu.update-manager' b'first-run' b'false'
 b'com.ubuntu.update-manager' b'window-width' b'757'
 b'com.ubuntu.update-manager' b'launch-time' b'int64 1542401633'
InstallationDate: Installed on 2015-04-29 (1297 days ago)
InstallationMedia: Ubuntu MATE 14.04.1 "Trusty Tahr" - final amd64 (20141111)
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: Upgraded to xenial on 2018-03-18 (243 days ago)