Section 12. The supreme judicial court, the superior court or the probate court may, upon petition of a party beneficially interested in a trust under a written instrument, and after notice to the trustee and all persons interested, remove the trustee if it finds that such removal is for the interests of the beneficiaries of the trust or if he has become incapacitated by reason of mental illness or otherwise incapable or is unsuitable therefor. If the petition for removal contains a prayer therefor, the court may, upon such notice as it considers reasonable, appoint a successor to fill the vacancy caused by such removal, without the filing of a separate petition for that purpose.