Clicking "Install" with .deb gives no response when APT in use

[Noor Muhammad Malik]

The problem description
========================
If I open up a .deb file to install the package, it opens up the gnome-software GUI window, with the install button.
If the package manager APT is in use somewhere, either you left an "apt upgrade" running in the background, or there's some APT process running in the background, meaning that the lock is acquired by another process, clicking on the "install" button will do nothing. It will get stuck on a random % of progress and then just do nothing, no installations, no errors.
If you let all the APTs finish, or kill existing processes to release the lock, the install button works fine.

What I expected to happen:
==========================
I expected it to check for a running instance of the package manager, check if the lock is free and then generate a pop-up/dialogue-box telling me that the package manager is in use, instead of randomly misbehaving with no output, which is bad user experience.

I'd recently encountered this bug and it was very frustrating for quite a while until I figured out why it was happening and wanted to report it.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-software 3.28.1-0ubuntu4.18.04.15
ProcVersionSignature: Ubuntu 4.15.0-108.109-generic 4.15.18
Uname: Linux 4.15.0-108-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat Jul 4 01:45:58 2020
InstallationDate: Installed on 2018-04-15 (810 days ago)
InstallationMedia: Ubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.2)
InstalledPlugins:
 gnome-software-plugin-flatpak N/A
 gnome-software-plugin-limba N/A
 gnome-software-plugin-snap 3.28.1-0ubuntu4.18.04.15
SourcePackage: gnome-software
UpgradeStatus: No upgrade log present (probably fresh install)