to revoke his probation following the filing of a notice of appeal. A court must always have jurisdiction to act. Thus, any action taken by a court when it lacks jurisdiction is a nullity. McLeod v. Provident Mutual Life Insurance Co., 186 Colo. 234, 526 P.2d 1318 (1974). As a general rule, when an appeal has been perfected, the trial court is divested of jurisdiction to issue any further orders in the case. Brooke v. People, 139 Colo. 388, 339 P.2d 993 (1959). However, there are recognized exceptions to this general rule. One such exception is that a trial court retains jurisdiction over proceedings that are either authorized by statute or rule or do not involve a challenge to the propriety of the judgment on appeal. See, e.g., In re Marriage of McCue, 645 P.2d 854 (Colo.App.1982) (<HOLDING>); see also Johnson v. District Court, 674 P.2d
holding that the appellate court should have construed the notice of appeal from the denial of a motion to vacate the judgment as an attempt to appeal from the underlying judgment holding that a trial court is not divested of its jurisdiction to hear an ocga  91514 motion pending appeal of the judgment below holding that an order pending divorce is not a final judgment that creates a right of appeal holding that the plaintiff was precluded from raising the issue for the first time on appeal recognizing that the trial court is not precluded from enforcing its judgment even though an appeal is pending