fix SimpleSession NPE

core/java/src/net/i2p/client/I2PSessionImpl.java

@@ -659,6 +659,8 @@ abstract class I2PSessionImpl implements I2PSession, I2CPMessageReader.I2CPMessa
                 propogateError("Error destroying the session", ipe);
             }
         }
+        // SimpleSession does not initialize
+        if (_availabilityNotifier != null)
             _availabilityNotifier.stopNotifying();
         _closed = true;
         _closing = false;

core/java/src/net/i2p/client/I2PSimpleSession.java

@@ -47,6 +47,7 @@ class I2PSimpleSession extends I2PSessionImpl2 {
      * @throws I2PSessionException if there is a problem
      */
     public I2PSimpleSession(I2PAppContext context, Properties options) throws I2PSessionException {
+        // Warning, does not call super()
         _context = context;
         _log = context.logManager().getLog(I2PSimpleSession.class);
         _handlerMap = new SimpleMessageHandlerMap(context);