cluster: prevent panic

Signed-off-by: xhe <[email protected]>

Author: xhebox

pkg/cluster/operation/upgrade.go

@@ -199,7 +199,9 @@ func Upgrade(
 				return err
 			}
 
-			waitFunc()
+			if waitFunc != nil {
+				waitFunc()
+			}
 		}
 
 		// process deferred instances