Java.lang.nullpointerException

public class DBConnection {
    private Connection conn;
    public  Connection ConnectBD(){
    try{
    Class.forName("com.mysql.jdbc.Driver");
        System.out.println("connection success");}
    catch(ClassNotFoundException cnfe){
        System.out.println("connection fail"+ cnfe);
    }
    
    String url="jdbc:derby://localhost:1527/firstDataBase";
   
    try{
    conn=(Connection) DriverManager.getConnection(url,"root", "");
        System.out.println("DataBase Connected");
    }
    
   catch(SQLException e){
       System.out.println("No connection "+e);
   }
    return conn;
    } 
}

 Connection conn= new DBConnection().ConnectBD();

private void loginActionPerformed(java.awt.event.ActionEvent evt) {                                      
      
     String sql="select * from APP.USER1 when USERNAME="+username.getText() +"and PASSWORD="+password.getText();
        try{
          PreparedStatement  pst=conn.prepareStatement(sql);
           
          ResultSet  rs=pst.executeQuery();
            if(rs.next()){
            JOptionPane.showMessageDialog(null, "welcome");
             Welcome w= new Welcome();
            w.setVisible(true);
            }
            else {
            JOptionPane.showMessageDialog(null,"user incorrect");
            }
                   
        }
        catch(Exception e){
            JOptionPane.showMessageDialog(null, e);
           
        }
    }