When using DB_CLOSE_DELAY, sometimes a NullPointerException is thrown.

h2/src/main/org/h2/engine/DatabaseCloser.java

@@ -34,7 +34,9 @@ public class DatabaseCloser extends Thread {
         synchronized (this) {
             databaseRef = null;
         }
-        if (getThreadGroup().activeCount() > 100) {
+        ThreadGroup threadGroup = getThreadGroup();
+        // the threadGroup could be null if the thread was run in the meantime
+        if (threadGroup != null && threadGroup.activeCount() > 100) {
             // in JDK 1.4 and below, all Thread objects are added to the ThreadGroup, 
             // and cause a memory leak if never started.
             // Need to start it, otherwise it leaks memory in JDK 1.4 and below