added null check

.settings/edu.umd.cs.findbugs.core.prefs

 #FindBugs User Preferences
-#Mon Jul 27 14:55:35 CEST 2015
+#Thu Jul 30 13:22:16 CEST 2015
 detector_threshold=2
 effort=max
 excludefilter0=.fbExcludeFilterFile|true

src/main/java/teetime/framework/AbstractRunnableStage.java

@@ -29,6 +29,10 @@ abstract class AbstractRunnableStage implements Runnable {
 	protected final Logger logger;
 
 	public AbstractRunnableStage(final Stage stage) {
+		if (stage == null) {
+			throw new IllegalArgumentException("Argument stage may not be null");
+		}
+
 		this.stage = stage;
 		this.logger = LoggerFactory.getLogger(stage.getClass());