Crash when using QConcatenateTablesProxyModel to handle QWebEnginePage::desktopMediaRequested under i3 window manager

I'm reporting this bug after investigating a bug report for Konqueror. Under special circumstances (described below), when using the i3 window manager, Konqueror crashes when displaying a dialog to choose the window or screen to share in response to the QWebEnginePage::desktopMediaRequested signal.

I attach an example program which has the same behavior. What it does is creating a dialog containing a combo box, then using a QConcatenateTablesProxyModel to concatenate the contents of three models: one is a `QStandardItemModel` with a single line, to be used as a default entry, saying "Choose a window or screen", the other two are the windowsModel() and screensModel() provided by the QWebEngineDesktopMediaRequest instance.

When using the i3 window manager, however, a crash occurs when doing the following:

• launch the program from an i3 workspace containing at least other 2 windows
• create a second workspace in i3 and open a single window in it
• go back to the first workspace
• in the example program, click the button which prompts the screen sharing, so that the dialog is shown
• select an entry corresponding to a window or screen in the combo box (that is, any entry except the default one) but do not press the Ok or Cancel button so that the dialog remains open
• switch to the second workspace
• the program crashes.

Note that the example program is hardcoded to display a MDN page about screen capture to test this crash: you need to scroll up a bit the page to find the button to start capturing the screen.

I also attach a backtrace of the crash in Konqueror. Unfortunately, I don't have a full backtrace with the example program because right now I don't have QtWebEngine with debug symbols installed right now.