[ubuntu] VirtualBox window disappears

I follow
Method 2: Install VirtualBox from the Oracle Repositories
https://linuxhint.com/install-virtualbox-linux/

performing following steps to install VirtualBox

$ wget -q https://www.virtualbox.org/download/..._vbox_2016.asc -O- | sudo apt-key add -

$ sudo add-apt-repository "deb [arch=amd64] http://download.virtualbox.org/virtualbox/debian $(lsb_release -cs) contrib"

$ sudo apt update

$ sudo add-apt-repository "deb [arch=amd64] http://download.virtualbox.org/virtualbox/debian $(lsb_release -cs) contrib"

$ sudo apt install virtualbox-6.1

Installation went through without problem.

On -> Activities -> VirtualBox
VirtualBox starts but its window disappears

On Terminal run
$ which virtualboxPlease help. Thanks

Regards