addPart.fxml中的fx:id="partTable"中没有元素.因此，partTable为空，并且

The is no element in addPart.fxml with fx:id="partTable". Consequently partTable is null, and

partTable.getItems();

引发空指针异常.

您需要将partTable注入到定义了它的FXML的控制器中:

You need to inject partTable into the controller for the FXML in which it is defined:

public class FXMLDocumentController implements Initializable {

    @FXML
    private Label label;

    @FXML
    private TableView<Inhouse> partTable ;

    // ...
}

AddPartController仅需要访问与表关联的项目列表，因此您可以为其定义一个字段以及用于初始化它的方法:

The AddPartController only needs access to the list of items associated with the table, so you can define a field for it, and a method for initializing it:

public class AddPartController implements Initializable {

    // ...

    private ObservableList<Inhouse> tableItems ;

    public void setTableItems(ObservableList<Inhouse> tableItems) {
        this.tableItems = tableItems ;
    }

    // ...
}

然后在加载addPart.fxml时设置项目:

Then set the items when you load addPart.fxml:

@FXML
private void addPart(ActionEvent event) throws IOException {

    FXMLLoader loader = new FXMLLoader(getClass().getResource("addPart.fxml"));
    Parent add_part_parent = loader.load();

    AddPartController addPartController = loader.getController();
    addPartController.setTableItems(partTable.getItems());

    Scene add_part_scene = new Scene(add_part_parent);
    add_part_scene.getStylesheets().add("style.css");
    Stage app_stage = (Stage) ((Node) event.getSource()).getScene().getWindow();
    app_stage.setScene(add_part_scene);
    app_stage.show();
}

然后您当然只需要

@FXML
public void addInhouse(ActionEvent event){

    tableItems.add(new Inhouse(partNameField.getText(),
            Integer.parseInt(partInstockField.getText()),
            Double.parseDouble(partPriceField.getText()),
            Integer.parseInt(partMaxField.getText()),
            Integer.parseInt(partMinField.getText()),
            Integer.parseInt(inhouseTextField.getText())
            //  Integer.parseInt(outsourcedTextField.getText())
            ));
}

(FWIW我不知道是什么

(FWIW I have no idea what

partInstockField.setText(String.valueOf(partInstockField));

等应该做的.)