Check if extras are set or not

Use the Intent.hasExtra(String name) to check if an extra with name was passed in the intent.

Example:

Intent intent = getIntent();

if (intent.hasExtra("bookUrl")) {
    bookUrl = b.getString("bookUrl");
} else {
   // Do something else
}

Also, use Intent.getStringExtra(String name) directly on the intent to handle the NullPointerException if no extras were passed.