In the event a contest of the probate of a will is instituted in the circuit court, as is or may be authorized by law, all parties interested in the probate of the will, as devisees, legatees or otherwise, as well as those interested in the testator if he had died intestate, as heirs, distributees or next of kin, shall be made parties to the contest; and if there be minors or persons of unsound mind interested in the estate or in the probate of the will, they shall be represented by their legal guardian, if such they have; if they have no such guardian, the court shall appoint an attorney-at-law as guardian ad litem to represent their interest in the contest, and the final judgment in such contest proceedings shall be conclusive as to all matters which were litigated or could have been litigated in such contest; and no further proceedings shall ever be entertained in any courts of this state to probate or contest the probate of such will.