Passing combobox value to classfile set failed

Object reference not set to an instance of an object

Form2 fr2 = new Form2();
                    
                     string srv = fr2.sqlServerComboBox.SelectedItem.ToString();
                     string pass = fr2.passwordTextBox.Text.ToString();
                     string database = fr2.databaseComboBox.SelectedItem.ToString();
                   

                   string str = @"Data Source=srv;" + "Initial Catalog=database;" + "Persist Security Info=True;" + "User ID=sa;" + "Password=pass";