Return an error if a mutator is nil

Author: dlipovetsky

cmd/clusterctl/client/cluster/mover.go

@@ -1329,7 +1329,13 @@ func applyMutators(object client.Object, mutators ...ResourceMutatorFunc) (*unst
 	}
 	u.SetUnstructuredContent(to)
 	for _, mutator := range mutators {
-		if err := mutator(u); err != nil {
+		var err error
+		if mutator != nil {
+			err = mutator(u)
+		} else {
+			err = fmt.Errorf("mutator is nil")
+		}
+		if err != nil {
 			return nil, errors.Wrapf(err, "error applying resource mutator to %q %s/%s",
 				u.GroupVersionKind(), object.GetNamespace(), object.GetName())
 		}