added null checks to timeline snapping and seekablewidget seeking

app/widget/timelinewidget/timelinewidget.cpp

@@ -1503,6 +1503,10 @@ QVector<SnapData> AttemptSnap(const QVector<double>& screen_pt,
 
 bool TimelineWidget::SnapPoint(QVector<rational> start_times, rational* movement, int snap_points)
 {
+  if (!GetConnectedNode()) {
+    return false;
+  }
+
   QVector<double> screen_pt;
 
   foreach (const rational& s, start_times) {

app/widget/timeruler/seekablewidget.cpp

@@ -160,6 +160,10 @@ int SeekableWidget::TimeToScreen(const rational &time) const
 
 void SeekableWidget::SeekToScreenPoint(int screen)
 {
+  if (timebase().isNull()) {
+    return;
+  }
+
   int64_t timestamp = qMax(static_cast<int64_t>(0), ScreenToUnitRounded(screen));
 
   if (Core::instance()->snapping() && snap_service_) {