I got this pop-up recently and while it must be from some desktop application that uses Electron.js, it’s not apparent which.

An easy way to tell is to run:

$ xprop

This will change the mouse pointer (to a cross-hair pointer on my Gnome 3 desktop), as the xprop invocation waits. Clicking with this on the target window will make xprop print a bunch of information about it, including the name of the main application window and its pid:

...
WM_CLASS(STRING) = "discord", "Discord"
WM_ICON_NAME(STRING) = "Error"
...