Convert nested map to nested list

Assume I have a class Level with nested Levels i.e.

public class Level {
    private Map<String, Level> childLevels
}

I want to convert this class into the following:

public class ListLevel {
    private List<ListLevel> childLevels
}

ChildLevels are not necessarily instantiated and may be null. I start with a Map<String, Level> and I want to convert this into a List<ListLevel> by discarding the key of every Map entry.

I've tried the code below but it's not quite there. edit: below code won't work with a new ListLevel class

private static List<Level> convertToList(Map<String, Level> levelMap){
    return levelMap.values().stream()
        .filter(i -> i.getChildLevels() != null)
        .flatMap(i -> Stream.concat(Stream.of(i), convertToList(i.getChildLevels()).stream()))
        .collect(Collectors.toList());
}

Any suggestions?

edit: to clarify, I want to convert all nested maps to lists but maintain the same nested structure

You need to use values() of Map like levelMap.values().stream() since you want the values of map.

private static List<Level> convertToList(Map<String, Level> levelMap){
    return levelMap.values().stream()
        .filter(i -> i.getChildLevels() != null)
        .flatMap(i -> Stream.concat(Stream.of(i), convertToList(i.getChildLevels()).stream()))
        .collect(Collectors.toList());
}

After question Updated:

private static List<ListLevel> convertToList(Map<String, Level> levelMap){
    if(levelMap == null) return null;
    return levelMap.values().stream()
        .map(i -> convertLevelToListLevel(i, convertToList(i.getChildLevels())))
        .collect(Collectors.toList());
}

Here ListLevel convertLevelToListLevel(Level level, List<ListLevel> childListLevels) is converter for Level to ListLevel conversion as mention in comment OP already have.