code: public final void addValue(@NonNull final String value) {
          Condition.INSTANCE.ensureNotNull(value, "The value may not be null");
  
          if (this.values != null) {
              if (this.values.add(value)) {
                  if (persistSet(this.values)) {
                      notifyChanged();
                  }
              }
          } else {
              Set<String> newValues = new HashSet<>();
              newValues.add(value);
              setValues(newValues);
          }
      }
docs: Adds a new value to the preference. By adding a value, the changes will be persisted.
  
  @param value
  The value, which should be added, as a {@link String}. The value may not be null