code: public void setDefaultInstrumentID(String id) {
      if (id == null) {
        unsetDefaultInstrument();
        return;
      }
  
      if (instruments.containsKey(id)) {
        defaultInstrumentID = id;
        isDefaultExplicitlySet = true;
      } else {
        throw new IllegalArgumentException(
            "The instrument map did not contain provided instrument ID, " +
                "have you added the instrument first?");
      }
    }
docs: Call with null parameter to unset.
  
  @param id this id must be present in the run info already.