Does a defendant who has entered a plea of guilty or no contest have to obtain a certificate of probable cause from the trial court to appeal?

Generally, a defendant who has entered a plea of guilty or no contest must obtain a certificate of probable cause from the trial court to appeal. (Pen. Code, 1237.5.) No certificate is required, however, when the grounds for appeal "arose after entry of the plea and do not affect the plea's validity." (Cal. Rules of Court, rule 8.304(b)(4)(B).) In determining whether a certificate is required, "the critical inquiry is whether a challenge to the sentence is in substance a challenge to the validity of the plea . . . ." (People v. Panizzon (1996) 13 Cal.4th 68, 76, italics omitted.)