In "VTrajectory", method "duration()" returns an "Optional<Double>" instead of...

In "VTrajectory", method "duration()" returns an "Optional<Double>" instead of "double" (required if trajectory is empty).

VadereState/src/org/vadere/state/simulation/VTrajectory.java

@@ -68,8 +68,14 @@ public class VTrajectory implements Iterable<FootStep> {
 		return clone().cut(startSimTime, endSimTime).length();
 	}
 
-	public double duration() {
-		return footSteps.peekLast().getEndTime() - footSteps.peekFirst().getStartTime();
+	public Optional<Double> duration() {
+		if(footSteps.isEmpty()) {
+			return Optional.empty();
+		}
+		else {
+			double duration = footSteps.peekLast().getEndTime() - footSteps.peekFirst().getStartTime();
+			return Optional.of(duration);
+		}
 	}
 
 	public Optional<Double> speed() {
@@ -77,7 +83,7 @@ public class VTrajectory implements Iterable<FootStep> {
 			return Optional.empty();
 		}
 		else {
-			return Optional.of(length() / duration());
+			return Optional.of(length() / duration().get());
 		}
 	}