code: @Nullable
      public static String optString(@Nullable Bundle bundle, @Nullable String key) {
          return optString(bundle, key, null);
      }
docs: Returns a optional {@link java.lang.String} value. In other words, returns the value mapped by key if it exists and is a {@link java.lang.String}.
  The bundle argument is allowed to be {@code null}. If the bundle is null, this method returns null.
  @param bundle a bundle. If the bundle is null, this method will return null.
  @param key a key for the value.
  @return a {@link java.lang.String} value if exists, null otherwise.
  @see android.os.Bundle#getString(String)