I found this
Showing and Hiding the NVL-mode Window
The NVL-mode window can be controlled with the standard window show and window hide statements. To enable this, add the following code to your game:
config.empty_window = nvl_show_core
config.window_hide_transition = dissolve
config.window_show_transition = dissolve
Setting config.empty_window to nvl_show_core will cause the NVL-mode window to be displayed during a transition. (The last two lines select the default transitions to be used for showing and hiding the window.)
But it doesn't seem to stop my NVL mode box from disappearing when I insert a new sprite into the image.
As an example, I have uploaded a set of screenshots of my problem.