javafx combobox selectionmodel is not consistant - javafx

Question

I am creating javafx application where I insert selected items of combobox which are also populated from database to another table of database but problem is that combobox's selection model is not working consistent some time it is giving item which are selected but some it sends null value to database
Code:
package application;
import java.net.URL;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ResourceBundle;
import org.controlsfx.control.textfield.TextFields;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.TableView;
import javafx.scene.control.TextField;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.scene.control.cell.PropertyValueFactory;
import javafx.scene.control.ComboBox;
import javafx.scene.control.DatePicker;
import javafx.scene.control.TableColumn;
public class EntryFormController implements Initializable {
Connection connection = SqliteConnection.Connector();
PreparedStatement preparedStatement = null;
ResultSet rs = null;
ObservableList<String> cBoxList1 = FXCollections.observableArrayList();
ObservableList<String> cBoxList2 = FXCollections.observableArrayList();
#FXML
private ComboBox<String> TruckNo;
#FXML
private ComboBox<String> exp1;
#FXML
private ComboBox<String> exp2;
#FXML
private void addButton(ActionEvent e) {
String query = "insert into entry (Truck,Expense)values(?,?),(?,?) ";
try {
preparedStatement = connection.prepareStatement(query);
preparedStatement.setString
(2,TruckNo.getSelectionModel().getSelectedItem());
preparedStatement.setString
(3, exp1.getSelectionModel().getSelectedItem());
TruckNo.getSelectionModel().getSelectedItem());
preparedStatement.setString
(9,exp2.getSelectionModel().getSelectedItem());
preparedStatement.execute();
TruckNo.setValue(null);
TruckNo.valueProperty().set(null);
exp1.setValue(null);
exp1.valueProperty().set(null);
exp2.setValue(null);
exp2.valueProperty().set(null);
preparedStatement.close();
} catch (SQLException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
}
#Override
public void initialize(URL location, ResourceBundle resources) {
// TODO Auto-generated method stub
exp1.setItems(cBoxList1);
exp1.setEditable(true);
loadDatabaseDataCombo1();
exp2.setItems(cBoxList2);
exp2.setEditable(true);
TruckNo.setItems(trkList);
TruckNo.setEditable(true);
loadDatabaseDataCombo6();
TextFields.bindAutoCompletion(exp1.getEditor(), exp1.getItems());
TextFields.bindAutoCompletion(exp2.getEditor(), exp2.getItems());
TextFields.bindAutoCompletion(TruckNo.getEditor(),TruckNo.getItems());
}
private void loadDatabaseDataCombo6() {
String query = "select * from Truck";
try {
preparedStatement = connection.prepareStatement(query);
rs = preparedStatement.executeQuery();
while (rs.next()) {
trkList.add(rs.getString("TruckNo"));
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}
private void loadDatabaseDataCombo2() {
String query = "select * from Expense";
try {
preparedStatement = connection.prepareStatement(query);
rs = preparedStatement.executeQuery();
while (rs.next()) {
cBoxList2.add(rs.getString("name"));
}
} catch (SQLException e) {
e.printStackTrace();
}
}
private void loadDatabaseDataCombo1() {
String query = "select * from Expense";
try {
preparedStatement = connection.prepareStatement(query);
rs = preparedStatement.executeQuery();
while (rs.next()) {
cBoxList1.add(rs.getString("name"));
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}