Testamentary guardian.) A parent of a person with a disability may designate by will a person, corporation or public agency qualified to act under Section 11a-5, to be appointed as guardian or as successor guardian of the person or of the estate or both of that person. If a conservator appointed under a prior law or a guardian appointed under this Article is acting at the time of the death of the parent, the designation shall become effective only upon the death, incapacity, resignation or removal of the conservator or guardian. If no conservator or guardian is acting at the time of the death of the parent, the person, corporation or public agency so designated or any other person may petition the court having jurisdiction over the person or estate or both of the child for the appointment of the one so designated. The designation shall be proved in the manner provided for proof of will. Admission of the will to probate in any other jurisdiction shall be conclusive proof of the validity of the designation. If the court finds that the appointment of the one so designated will serve the best interests and welfare of the ward, it shall appoint the one so designated. The selection of a guardian shall be in the discretion of the court, whether or not a designation is made.