Issue #604 reports that crashes are possible in playback window if someone clicks play

when nothing is loaded to playback. Instead now a warning is shown.

frameplaybackwindow.h

@@ -69,6 +69,7 @@ private:
     void readSettings();
     void writeSettings();
     void calculateWhichBus();
+    bool checkNoSeqLoaded();
     bool eventFilter(QObject *obj, QEvent *event);
 };