QT application window not shown when started directly with xinit as non-root user

[qt_slackware]

For example:

xinit /usr/bin/VirtualBox  -- :0 vt1
xinit /usr/bin/kdevelop  -- :0 vt1

In both cases the main window does not show up and screen stays blank. Only the initial window, for example "One or more disk image files are not currently accessible. As a result, you will not be able to operate virtual machines that use these files until they become accessible later." shows up. This however makes the main window show up:

sudo xinit /usr/bin/VirtualBox  -- :0 vt1
sudo xinit /usr/bin/kdevelop  -- :0 vt1

If xterm is started instead one also needs to use sudo for the main windows to show up.

I'm using Slackware Linux but the same behavior was reported on Arch here: https://unix.stackexchange.com/questions/410233/arch-linux-running-virtualbox-without-a-window-manager.