NPE check to avoid crashes

[mattshen]

The NPE causes a lot of crashes in our app. Although we had JS check to avoid shutting down un-initialized client as follow:

• create client

    let clientInitialised = false;

    client.initialize().then(() => {
      clientInitialised = true;
      accessManager.registerClient();
    }).catch((error) => {
      console.log(error);
    });

• shutdown client

if (client && clientInitialised) {
   client.shutdown();
}

But it seems there are still some edge cases we don't cover and it's hard to reproduce in our test env. So we decided to add this nice-to-have NP check before shutting the client in Java code.