JavaFx: multiple controllers

I'm working on the project which contains multiple controllers (1 main and two for each of the Fxml files). Unfortunately while running the program, console throw the NullPointerExeption at me. I found out that it's the main controller fault, but still having this information I'm unable to fix this. Could you give me some tips how to solve this issue ? Here i got the full track:

MainController class:

public class MainController {
  @FXML LogInController logInController;
  DBConnect dbConnect;
  @FXML FlightControlController flightControlController;

  @FXML public void initialize() {
    logInController.initialize(this);
    flightControlController.initialize(this);
    dbConnect.initialize(this);
  }
}

DBConnect class:

public void dbConnect() throws SQLException {
  try {
    Conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/nasa", "root", "1234");
  } catch(Exception e) {
    System.out.println(e.getStackTrace());
  }
}

public boolean isCorrect(String login,String password) throws SQLException, IOException {
  Stmt = Conn.createStatement();
  Rs = Stmt.executeQuery("SELECT * FROM Users WHERE login='"+login+"'AND password='"+password+"';");
  if(Rs.next()) {
    return true; 
  } else {
    return false;
  }
}

public void initialize(MainController mainController) {
  this.mainController=mainController;
}

LogInController class:

MainController mainController;
@FXML DBConnect dbConnect;
@FXML TextField logginField = null;
@FXML PasswordField passwordFiled=null;
@FXML Button logInButton;

@FXML
public void buttonClicked(ActionEvent event) throws SQLException, IOException {
  mainController.dbConnect.dbConnect();
  if(mainController.dbConnect.isCorrect(logginField.getText(),passwordFiled.getText())) {
    Parent root = FXMLLoader.load(getClass().getResource("/view/FlightControlView.fxml"));
    Scene scene = new Scene(root);
    Stage stage = (Stage) ((Node) event.getSource()).getScene().getWindow();
    stage.setScene(scene);
    stage.show();
  } else {
    System.out.print("An error have occured");
  }
}

public void initialize(MainController mainController) {
  this.mainController=mainController;
}

FlightControlController Class:

@FXML Label label;
MainController mainController;

public void initialize(MainController mainController) {
  this.mainController = mainController;
}

And the error which occurs:

Caused by: java.lang.NullPointerException at controller.LogInController.buttonClicked(LogInController.java:31) ... 62 more

I'm not sure but is it possible that MainController (which doesn't have his own Fxml file) so that FXMLLoader doesn't initialize MainController's properties?

@EDIT

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.text.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>

<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="693.0" prefWidth="1062.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="controller.LogInController">
   <children>
      <SplitPane dividerPositions="0.21320754716981133" layoutX="331.0" layoutY="220.0" prefHeight="693.0" prefWidth="1062.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
        <items>
         <AnchorPane minHeight="0.0" minWidth="0.0" prefHeight="160.0" prefWidth="100.0">
           <children>
                  <Label alignment="CENTER" layoutY="-1.0" maxHeight="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="70.0" prefWidth="222.0" text="NASA DATABASE" />
                  <TextField fx:id="logginField" layoutX="18.0" layoutY="101.0" promptText="Login" />
                  <PasswordField fx:id="passwordFiled" layoutX="18.0" layoutY="165.0" promptText="Password" />
              <Button fx:id="logInButton" onAction="#buttonClicked" layoutX="79.0" layoutY="230.0" mnemonicParsing="false" text="Log In" />
           </children>
        </AnchorPane>
      <AnchorPane prefHeight="691.0" prefWidth="493.0" SplitPane.resizableWithParent="false">
           <children>
              <Label alignment="CENTER" layoutX="97.0" layoutY="104.0" prefHeight="417.0" prefWidth="635.0" text="NOT AVAILABLE, PLEASE LOG IN" AnchorPane.leftAnchor="113.0" AnchorPane.rightAnchor="113.0">
                 <font>
                    <Font name="System Bold Italic" size="37.0" />
                 </font>
              </Label>
           </children>
        </AnchorPane>
    </items>
  </SplitPane>
   </children>
</AnchorPane>