HPLIP missing packages

Question

Ubuntu 18.04.1, HPLIP 3.17.10.

I am trying to install HPLIP using Synaptic. The installation reports no errors. But, when I start the HP system tray, I get this error message:

File "/usr/bin/hp-toolbox", line 269, in <module>
     QApplication, ui_package = utils.import_dialog(ui_toolkit)
 TypeError: 'NoneType' object is not iterable

Also, the HP icon in the tray area does nothing if I click on it - although I am able to start the HPLIP Toolbox, and that seems to work.

When I run hp-check, it tells me that the following packages are missing/incompatible, and that I need to install them manually:

libnetsnmp-devel
snmp-mibs-downloader
libsnmp-dev

I do not know where to find the first two, nor how to install them manually.

Regarding libsnmp-dev: It is not installed, but via Synaptic versions 5.7.3+dfsg-1.8ubuntu3 and 5.7.3+dfsg-1.8ubuntu3.1 are available to install. However, when I try to install libsnmp-dev via Synaptic, it tells me that the following packages will be removed - but I don't want them to be removed (especially npm):

libssl1.0-dev
node-gyp
nodejs-dev
npm

How do I get the missing packages installed, get HPLIP working, get rid of the error message (above), and make the HP system tray to work, without uninstalling npm and those other packages?

Output of the apt-cache policy libssl1.0-dev node-gyp nodejs-dev npm:

libssl1.0-dev:
  Installed: 1.0.2n-1ubuntu5.2
  Candidate: 1.0.2n-1ubuntu5.2
  Version table:
 *** 1.0.2n-1ubuntu5.2 500
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1.0.2n-1ubuntu5 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages

node-gyp:
  Installed: 3.6.2-1ubuntu1
  Candidate: 3.6.2-1ubuntu1
  Version table:
 *** 3.6.2-1ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        500 http://archive.ubuntu.com/ubuntu bionic/universe i386 Packages
        100 /var/lib/dpkg/status

nodejs-dev:
  Installed: 8.10.0~dfsg-2ubuntu0.4
  Candidate: 8.10.0~dfsg-2ubuntu0.4
  Version table:
 *** 8.10.0~dfsg-2ubuntu0.4 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/universe amd64 Packages
        100 /var/lib/dpkg/status
     8.10.0~dfsg-2ubuntu0.2 500
        500 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages
     8.10.0~dfsg-2 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages

npm:
  Installed: 3.5.2-0ubuntu4
  Candidate: 3.5.2-0ubuntu4
  Version table:
 *** 3.5.2-0ubuntu4 500
        500 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        500 http://archive.ubuntu.com/ubuntu bionic/universe i386 Packages
        100 /var/lib/dpkg/status

Output of sudo apt install libsnmp-dev:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  gyp libjs-async libjs-inherits libjs-node-uuid libuv1-dev node-abbrev
  node-ansi node-ansi-color-table node-archy node-async node-balanced-match
  node-block-stream node-brace-expansion node-builtin-modules
  node-combined-stream node-concat-map node-cookie-jar node-delayed-stream
  node-forever-agent node-form-data node-fs.realpath node-fstream
  node-fstream-ignore node-github-url-from-git node-glob node-graceful-fs
  node-hosted-git-info node-inflight node-inherits node-ini
  node-is-builtin-module node-isexe node-json-stringify-safe node-lockfile
  node-lru-cache node-mime node-minimatch node-mkdirp node-mute-stream
  node-node-uuid node-nopt node-normalize-package-data node-npmlog node-once
  node-osenv node-path-is-absolute node-pseudomap node-qs node-read
  node-read-package-json node-request node-retry node-rimraf node-semver
  node-sha node-slide node-spdx-correct node-spdx-expression-parse
  node-spdx-license-ids node-tar node-tunnel-agent node-underscore
  node-validate-npm-package-license node-which node-yallist    
Use 'sudo apt autoremove' to remove them.    
The following additional packages will be installed:    
  libpci-dev libsensors4-dev libssl-dev libssl-doc libudev-dev libwrap0-dev    
The following packages will be REMOVED:
  libssl1.0-dev node-gyp nodejs-dev npm
The following NEW packages will be installed:
  libpci-dev libsensors4-dev libsnmp-dev libssl-dev libssl-doc libudev-dev
  libwrap0-dev
0 upgraded, 7 newly installed, 4 to remove and 7 not upgraded.
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