"Software Sources" fails to launch from Discover when tried to launch from applications list (kf.kio.gui: "Could not find the program 'lxqt-sudo'")

[Sai Vinoba]

Release: Kubuntu Groovy (also affects focal, hirsute)
SW, version: software-properties-qt, 0.99.3.1 (for groovy but also affects version on focal)

How to reproduce:
1) Launch Discover.
2) Search for (or scroll to) 'software sources', select the 'Software Sources' from the search result.
3) Click on 'Launch'.

Expected:
The application launches, possibly asking for admin password.

Actual:
Does not launch (on groovy). On focal, launches with error (Snapshot attached). However, if we try to access software sources from 'Settings', it launches properly.

When launched from terminal, it was seen that discover was trying to use `lxqt-sudo` and failing since it was not installed. Error message as below.
`kf.kio.gui: "Could not find the program 'lxqt-sudo'"`

Whereas when we access 'Software Sources' from 'Settings', it is using `kdesu`. Message as below.
`org.kde.kdesu: Daemon not safe (not sgid), not using it.`. Inspite of 'Daemon not safe...' error, the software sources launches and able to make changes.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: software-properties-qt 0.99.3.1
ProcVersionSignature: Ubuntu 5.8.0-53.60-generic 5.8.18
Uname: Linux 5.8.0-53-generic x86_64
ApportVersion: 2.20.11-0ubuntu50.7
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Fri May 28 11:41:34 2021
InstallationDate: Installed on 2021-05-28 (0 days ago)
InstallationMedia: Kubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)

[Sai Vinoba (saivinob)]

On Groovy, Hirsute, no error message window pops-up; the 'Launch' button is not-responsive. On Focal an error message is shown. Snap attached.

[Sai Vinoba (saivinob)]

If tried to access from 'Settings', `kdesu` is used instead of `lxqt-sudo` and application starts properly. Snap attached.