Subject to Section 1252, evidence of a statement of the declarant’s state of mind, emotion, or physical sensation (including a statement of intent, plan, motive, design, mental feeling, pain, or bodily health) at a time prior to the statement is not made inadmissible by the hearsay rule if:
(a) The declarant is unavailable as a witness; and
(b) The evidence is offered to prove such prior state of mind, emotion, or physical sensation when it is itself an issue in the action and the evidence is not offered to prove any fact other than such state of mind, emotion, or physical sensation.