TriggerAutoSaveOnBackground can cause crashes!

[haochiz]

This function is executed when the app has been in the background over a specified duration, it saves the note and end audio (if audio running).

However, if the app is on the NoteOverViewPage, a 'System.NullReferenceException' exception is thrown because the current note is 'null', if a note was never opened this exception will cause a crash: "An exception of type 'System.NullReferenceException' occurred in PhenoPad.exe but was not handled in user code".