So that you're not getting into a state of viewing an invalid index. If you switch to a printer with 0 scripts attached, the index is still invalid, but it won't display anything then so you're okay.
This causes the button to display immediately on start-up if there were any post-processing scripts in the current machine. If there weren't, then no button will be shown (as specified in the QML).
Previously the QML was only loaded first upon clicking the item in the extensions menu. Now it has to be loaded on start-up in order to display that button.
