Benefit to widow, next of kin, designated beneficiary, or estate. Upon the death of any contributor or pensioner, the board of trustees shall pay an amount equal to a refund of contributions as provided in Section 19-215 of this Division: (a) to the widow of such deceased contributor or pensioner, or (b) if there be no widow then to the children, under the age of 18, of such deceased contributor or pensioner, share and share alike, or (c) if there be no widow or children under the age of 18, such amount may be paid to a beneficiary designated in writing to the board of trustees by such contributor or pensioner, or (d) if there be no widow, or children under the age of 18, or designated beneficiary, or if there be a beneficiary designated and such beneficiary shall predecease the contributor or pensioner, then such sum shall be paid to the executor or administrator of the estate of such deceased person provided, that if the estate of such contributor or pensioner shall come within the provisions of Article XXV of the Probate Act of 1975, as amended, such amount may be paid out under the provisions of said Article; and further provided, that wherever any benefits have been paid to a contributor or pensioner during his life time, the amount paid under this section shall be decreased by the total amount previously paid to such contributor or pensioner.