How do I convert a TreeMap that is a <String, <List<String>> to String[][]?

I'm not sure what I should initialize my array String [][] ans to be.

If I set it to be the size of the treeMap then there's a NPE.

String [][] ans = new String[size][];

So I initialized to be bigger than expected but then there are nulls in my array.

Currently my treeMap is {C=[E], M=[F, B], S=[F, B], T=[F, B, L], TD=[E, L], W=[E, L], Z=[F]}

I want it to be

[[C,E], [M, F, B], [S, F, B]....]

import java.util.*;

public class Copy {
    public static String[][] oMI(String[][] m){

        //ans[0][0]= "test";
        Map<String, List<String>>  map  = new HashMap<>();
        int length = m.length;
        for (int i=0; i < m.length;i++){
            for (int j=1; j < m[i].length;j++){
                String key = m[i][j];

                map.computeIfAbsent(key, k-> new ArrayList<String>()).add(m[i][0]);
            }
        }
        Map<String,List<String>> treeMap = new TreeMap<String, List<String>>(map);
        System.out.println(treeMap);

        int size = treeMap.size();

        String [][] ans = new String[20][20];
        int l = 0;
        int s = 0;
        String ing = "";
        for (Map.Entry<String,List<String>> entry: treeMap.entrySet()){
            s = entry.getValue().size();

            if (entry.getKey() !=null)
                ing = entry.getKey();
            else ing ="null";

            ans[l][0] = ing;

            int mn = 0;
            for (String v : entry.getValue()) {
                ans[l][mn] = v;       
                l++;
                mn++;
            }
        }
        return ans;
    }

    public static void main(String[] args){

        String  [][] ma =
                {
                        {"F", "T", "Z", "S", "M"},
                        {"E", "C", "W", "TD"},
                        {"B", "M", "S", "T"},
                        {"L", "W", "T", "TD"}

                };
        oMI(ma);
    }
}