How to write a binary search tree to a text file

I'm trying to write a binary Search tree to a text file in sorted order, my write method is in my BST class. When I print out the BST in Order Tranversal it works perfectly. When I try to write it to a text file it works but the current node being written inserts null to the text file instead of the node

public void write(Node focusNode)
{
    
    try{
        // Creates a FileWriter
        FileWriter fileName = new FileWriter("Binary Search Tree.txt");

        // Creates a BufferedWriter
        BufferedWriter  bw = new BufferedWriter(fileName);
        if (focusNode != null) {
            bw.write(focusNode.leftChild+"\n"); //adds the  left node

            bw.write(focusNode+"\n");//add the current node

            bw.write(focusNode.rightChild+"\n"); // add the right node
        }
        else{
            System.out.print("Your binary search tree is empty");
        }
        bw.close();
    }   
    catch(Exception ex) {
        System.out.print("An error occured try again ");
    }