What is the test for obtaining a certificate of probable cause to appeal a motion of appeal?

Without obtaining a certificate of probable cause (Pen. Code, 137.5), defendant filed a notice of appeal indicating the appeal (1) is based on the sentence or other matters occurring after the plea that do not affect the plea's validity, and (2) challenges the validity of the plea or admission. This court appointed counsel to represent defendant on appeal. Appointed counsel filed a brief raising no issues, but requested this court to independently review the record for arguable contentions pursuant to People v. Wende (1979) 25 Cal.3d 436. Defendant was advised by letter from this court of his right to file a supplemental brief within 30 days.