Cannot Invoke JTable because tableDisplay is null

Question

Hello I encountered a NullPointerException JTable error

Anyway this is the showUser()

public void showUser() {
    ArrayList<User> list = userList();
    DefaultTableModel model = (DefaultTableModel) tableDisplay.getModel();
    Object[] row = new Object[11];
    for (int i = 0;i<list.size();i++) {
        row[0] = list.get(i).getID();
        row[1] = list.get(i).getFname();
        row[2] = list.get(i).getMname();
        row[3] = list.get(i).getLname();
        row[4] = list.get(i).getDOB();
        row[5] = list.get(i).getAge();
        row[6] = list.get(i).getCity();
        row[7] = list.get(i).getGender();
        row[8] = list.get(i).getYear();
        row[9] = list.get(i).getSem();
        row[10] = list.get(i).getCourse();
        model.addRow(row);
    }
}

And here is the code of tableDisplay

tableDisplay = new JTable();
    scrollTable.setViewportView(tableDisplay);
    tableDisplay.setModel(new DefaultTableModel(
        new Object[][] {
        },
        new String[] {
            "Student ID", "First name", "Middle name", "Last name", "Birth Date", "Age", "City", "Gender", "Year", "Sem", "Course Code"
        }
    ));
    tableDisplay.getColumnModel().getColumn(5).setPreferredWidth(37);
    tableDisplay.getColumnModel().getColumn(6).setPreferredWidth(71);
    tableDisplay.getColumnModel().getColumn(8).setPreferredWidth(56);
    tableDisplay.getColumnModel().getColumn(9).setPreferredWidth(34);
    tableDisplay.getColumnModel().getColumn(10).setPreferredWidth(72);

I am new to programming and I would love some help.