Fixed error in toString method of stopwatch

pom.xml

@@ -26,6 +26,12 @@
             <artifactId>commons-math3</artifactId>
             <version>3.6.1</version>
         </dependency>
+        <dependency>
+            <groupId>org.jetbrains</groupId>
+            <artifactId>annotations</artifactId>
+            <version>RELEASE</version>
+            <scope>compile</scope>
+        </dependency>
     </dependencies>
 
 

src/main/java/quantization/utilities/Stopwatch.java

@@ -39,6 +39,9 @@ public class Stopwatch {
     }
 
     public String getElapsedTimeString() {
+        if (elapsed == null) {
+            return "No time measured yet.";
+        }
         return String.format("%dH %dMin %d Sec %d Ms", elapsed.toHoursPart(), elapsed.toMinutesPart(), elapsed.toSecondsPart(), elapsed.toMillisPart());
     }