Random Presentation Topic Picker returns null

Today my teacher asked me to program a random presentation topic picker for him.

The idea was, that the student goes to the pc and clicks on the message Dialog which then randomly generates a number between 1 and the max index of the topics and then prints the according topic.

I tried it with HashMaps. to put in the key that stays with the String together so that I can then (after the output) remove that entry so that no other student can get the same topic.

But it always returns at least 1 empty reference -> null.

Can anyone help me? Please :)

thats the code:

static HashMap<Integer, String> map = new HashMap<>();

public static void main(String[] args){

    int anzahlEintraege = Integer.parseInt(JOptionPane.showInputDialog("Wie viele Themen gibt es?"));

    for(int i = 0; i < anzahlEintraege; i++){
        map.put((i+1),JOptionPane.showInputDialog("Geben Sie das Thema Nummer " + (i+1) + " ein!"));
    }

    JOptionPane.showMessageDialog(null, "Jetzt geht's Los!");

    int max = map.size();
    int removed = 0;
    for(int i = 0; i < max; i++){
        Random r = new Random();
        int random = r.nextInt(max-1)+1;

        JOptionPane.showMessageDialog(null, "Sie haben das Thema "+ map.get(random) + " gezogen!");
        map.remove(random);
        removed++;

    }
}