Fix QML errors in InstrumentsTreeItemDelegate

- `popup` is not defined (anymore), because that's `openedPopup` now
- `openedPopup` is readonly, because it is an alias to `Loader.item`. Trying to set it to `null` doesn't make sense and causes an error.

src/instrumentsscene/qml/MuseScore/InstrumentsScene/internal/InstrumentsTreeItemDelegate.qml

@@ -147,14 +147,13 @@ FocusableControl {
             openedPopup.load(item)
 
             openedPopup.opened.connect(function() {
-                root.popupOpened(popup.x, popup.y, popup.height)
+                root.popupOpened(openedPopup.x, openedPopup.y, openedPopup.height)
             })
 
             openedPopup.closed.connect(function() {
                 root.popupClosed()
 
                 Qt.callLater(function() {
-                    openedPopup = null
                     popupLoader.sourceComponent = null
                 })
             })