[VirtualBox] How to fix "no host-only adapter is currently selected"

Problem Description

When I tried running a virtual machine of a Linux on which Mininet installed with VirtualBox, I had a trouble with choosing a secondary Host-only Adapter for SSH connections. There was a pop-up message, "No host-only network adapter is currently selected". 

The thing was that there was actually no network adapter I could select.

Solution

This happened because there was no virtual network adapter you could utilize as a host-only network device. Therefore, you should add them at the network tab in the VirtualBox's Preference pane. 

Then, you can select an adapter for the host-only use.