What happens when activating a window should be left to the window manager

KWin allows the user to choose between moving the window to the current desktop and switching to the window's desktop

Manually specifying the desktop here breaks this

It's always empty here, so no point in reading it

Use the correct API

Create a new startup id instead of trying to read an existing one, which doesn't work

Use KWindowSystem::activateWindow to activate the window instead of doing it implicitly via KStartupInfo::setNewStartupId