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.cpp

@@ -576,6 +576,7 @@ void FramePlaybackWindow::btnLoadLive()
 
 void FramePlaybackWindow::btnBackOneClick()
 {
+    if (!checkNoSeqLoaded()) return;
     forward = false;
     isPlaying = false;
     wantPlaying = false;
@@ -593,6 +594,7 @@ void FramePlaybackWindow::btnPauseClick()
 
 void FramePlaybackWindow::btnReverseClick()
 {
+    if (!checkNoSeqLoaded()) return;
     forward = false;
     wantPlaying = true;
     if (!ui->ckWaitForTraffic->isChecked())
@@ -628,6 +630,7 @@ void FramePlaybackWindow::btnStopClick()
 
 void FramePlaybackWindow::btnPlayClick()
 {
+    if (!checkNoSeqLoaded()) return;
     forward = true;
     wantPlaying = true;
     if (!ui->ckWaitForTraffic->isChecked())
@@ -640,6 +643,7 @@ void FramePlaybackWindow::btnPlayClick()
 
 void FramePlaybackWindow::btnFwdOneClick()
 {
+    if (!checkNoSeqLoaded()) return;
     forward = true;
     isPlaying = false;
     wantPlaying = false;
@@ -647,6 +651,16 @@ void FramePlaybackWindow::btnFwdOneClick()
     updateFrameLabel();
 }
 
+bool FramePlaybackWindow::checkNoSeqLoaded()
+{
+    if (seqItems.count() == 0)
+    {
+        QMessageBox::warning(this, "Warning", "Cannot begin playback until at\nleast one playback source is loaded.");
+        return false;
+    }
+    return true;
+}
+
 void FramePlaybackWindow::changePlaybackSpeed(int newSpeed)
 {
     playbackObject.setPlaybackInterval(newSpeed);

frameplaybackwindow.h

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