HPLIP missing packages

Ubuntu 18.04.1, HPLIP 3.17.10.

I am attempting 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 follwoing 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

So, I need some help here: 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?