[process] On Linux, when Theia is closed from the UI, its process is not killed

Description

On Linux, when Theia is run from the terminal and the application gets closed by the user from the UI, the Theia process is kept alive. Ctrl C is needed to kill the process.

Reproduction Steps

Try to compile and run Theia on desktop (on Linux) by using yarn start and then click on the X icon to close the program. The process in the Terminal is not killed.

OS and Theia version:

Diagnostics:

Not sure if referred to the main Theia process or to subprocess but this is the output printed when Theia gets closed from the UI:

process WARN This process was not registered or was already unregistered. [ID: 0]