doks: fix panic during create

If a user creates a DOKS cluster through doctl and deletes it through
some other means while doctl is waiting for the creation the command
panics. This fixes the panic and returns an appropriate error.

commands/kubernetes.go

@@ -764,6 +764,9 @@ func (s *KubernetesCommandService) RunKubernetesClusterCreate(defaultNodeSize st
 			if err != nil {
 				warn("Cluster couldn't enter `running` state: %v", err)
 			}
+			if cluster == nil {
+				return errors.New("cluster vanished while waiting for creation")
+			}
 		}
 
 		if update {