(a) At the trial, the proponents of the will have the burden of proof of due execution. The contestants of the will have the burden of proof of lack of testamentary intent or capacity, undue influence, fraud, duress, mistake, or revocation. If the will is opposed by the petition for probate of a later will revoking the former, it shall be determined first whether the later will is entitled to probate.
(b) The court shall try and determine any contested issue of fact that affects the validity of the will.