I got following controller for my application:
public class Controller {
    @FXML
    private CustomerPane customerPane;
}
The customerPane is a extended GridPane:
public class CustomerPane extends GridPane {
    @FXML
    private TableView<Customer> customerTable;
    public CustomerPane() {
        System.out.println(this.customerTable);
    }
}
My *.fmxl looks like (I left out some details):
<?import vm.CustomerPane?>
<CustomerPane fx:id="customerPane" layoutX="316.0" prefHeight="654.0" prefWidth="536.0" style="-fx-background-color: #7C8184;">
    <children>
        <TableView fx:id="customerTable" editable="true" prefHeight="200.0" prefWidth="461.0" GridPane.columnSpan="2" GridPane.rowIndex="1">
        </TableView>
    </children>
</CustomerPane>
I got two problems here:
- If I save the fmxl-file with a scene builder, it will override the import of my CustomerPane. Why is this the case and how can I fix it? 
- The - customerTableleads to null. Why and how can I fix it?
 
    