Fix #27201 - PLUGINS: methods returning pntrs to QObject lead to crash.

Fix #27201 - PLUGINS: methods returning pntrs to QObject lead to crash.

For details, including an analysis of the problem, see http://musescore.org/en/node/27201

This fix only covers `Q_INVOKABLE` methods in the `Note` class; there are probably more of them on other classes.