Fix NPE in TestRunSession#toString

currently if one calls TestRunSession#toString (e.g. in a debugger) a
NPE can occur if the session is not yet started and the fStartTime is
null.

This checks for this case and adjust the message accordingly and also
replace the volatile boolean with an AtomicBoolean.

debug/org.eclipse.unittest.ui/src/org/eclipse/unittest/internal/model/TestRunSession.java

@@ -23,6 +23,7 @@
 import java.util.Date;
 import java.util.HashMap;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicReference;
 
 import org.eclipse.unittest.internal.UnitTestPlugin;
 import org.eclipse.unittest.internal.launcher.TestViewSupportRegistry;
@@ -78,7 +79,7 @@ public class TestRunSession extends TestSuiteElement implements ITestRunSession,
 	 */
 	private HashMap<String, TestElement> fIdToTest;
 
-	volatile Instant fStartTime;
+	private final AtomicReference<Instant> fStartTime = new AtomicReference<>();
 	volatile Integer fPredefinedTestCount;
 
 	volatile boolean fIsAborted;
@@ -105,7 +106,7 @@ public TestRunSession(String testRunName, Instant startTime, ILaunchConfiguratio
 		fIdToTest = new HashMap<>();
 
 		fTestRunnerClient = null;
-		fStartTime = startTime;
+		fStartTime.set(startTime);
 
 		fSessionListeners = new ListenerList<>();
 	}
@@ -254,7 +255,7 @@ public int getCurrentIgnoredCount() {
 	 * @return an {@link Instant} object indicating a test run session start time
 	 */
 	public Instant getStartTime() {
-		return fStartTime;
+		return fStartTime.get();
 	}
 
 	/**
@@ -381,7 +382,8 @@ private class TestSessionNotifier {
 		 * @param testCount number of tests in this run
 		 */
 		public void testRunStarted(Integer testCount) {
-			fStartTime = Instant.now();
+			// only update if not already set!
+			fStartTime.compareAndSet(null, Instant.now());
 			fPredefinedTestCount = testCount;
 
 			for (ITestSessionListener listener : fSessionListeners) {
@@ -590,7 +592,12 @@ private void addFailures(Collection<TestElement> failures, TestElement testEleme
 
 	@Override
 	public String toString() {
-		return fTestRunName + " " + DateFormat.getDateTimeInstance().format(new Date(fStartTime.toEpochMilli())); //$NON-NLS-1$
+		Instant startTime = getStartTime();
+		if (startTime == null) {
+			return fTestRunName + " (not started)"; //$NON-NLS-1$
+		} else {
+			return fTestRunName + " " + DateFormat.getDateTimeInstance().format(new Date(startTime.toEpochMilli())); //$NON-NLS-1$
+		}
 	}
 
 	@Override