Testamentary additions to trusts.) By a will signed and attested as provided in this Act a testator may bequeath or appoint real and personal estate to a trustee of a trust evidenced by an instrument, including the will of another who predeceases the testator, which is in existence when the testator’s will is made and which is identified in the testator’s will, even though the trust is subject to amendment, modification, revocation or termination. Unless the testator’s will provides otherwise, the estate so bequeathed or appointed shall be governed by the terms and provisions of the instrument creating the trust, including any amendments or modifications in writing made at any time before or after the execution of the testator’s will and before, or after if the testator’s will so directs, the death of the testator. The existence, size or character of the corpus of the trust is immaterial to the validity of the bequest. If the trust is terminated prior to the testator’s death by revocation of the trust or by revocation of that portion of the instrument creating the trust, the bequest or appointment shall take effect according to the terms and provisions of the instrument creating the trust as they existed at the time of the termination, unless the testator’s will otherwise provides.