Binary tree not clearing with each loop iteration

Question

I am using a binary tree to convert prefix expression into infix, and then solving them. That part works fully.

My issue is the input loop I set up is adding on the new expression to the old tree, resulting in an error. I have tried clearing the linked list, and manually created a method to empty out the tree. But every time, the output I have is showing the new expression appended to the previous one, rather than creating a whole new tree.

    public static void main(final String[] args) {
        Scanner listener = new Scanner(System.in);
        System.out.println();
        System.out.println("Enter a prefix expression Separated by space!! (Enter 'done' to end): ");
        String line = listener.nextLine();
        while(!line.equalsIgnoreCase("done")){
        System.out.println(line);
            if(line.length()<=1){ System.out.println("Invalid expression"); listener.close(); return;}

            instances(line);
            line =  new String();
            line = listener.nextLine();
        }
        listener.close();
    }
    public static void instances(String input){
        System.out.println("The input expression was \n\t"+ input +"\n");
        String[] in = input.split(" ");
        LinkedList<String> queue = new LinkedList<>(Arrays.asList(in));
        TreeNode root = new TreeNode(queue, null);
        System.out.println("Expression as infix: ");
        inOrder(root);
        System.out.println(infix_string);
        String[] infix = infix_string.split(" ");
        Stack infix_stack = new Stack(infix);
        System.out.print("\t" + infix_stack + " = ");
        infix_stack.answer();
        System.out.println();
        System.out.println("Enter a prefix expression Separated by space!! (Enter 'done' to end): ");
        queue.clear();
        clearTree(root);
    }

    public static void clearTree(TreeNode root){
        if(root != null){
            clearTree(root.left);
            clearTree(root.right);
            root=null;
        }
    }
}