JavaFX TableView - 空指针异常

Question

我有一个无法加载我为其创建的“MediaFile”对象的TableView。无论我使用哪种（不同的单元工厂和单元值facotries）方法来显示MediaFile对象。当我调用playListTableView.setItems(DBUtils.getList());时，我得到一个空指针异常。这个MediaFile对象列表不为空 - 我可以看到它打印到控制台。任何有关解决这个问题的建议都会非常感激。

public class AudioPlayerFXMLController implements Initializable {

@FXML
private ListView playList;
@FXML
public static TableView<MediaFile> playListTableView;

@FXML
private TableColumn<MediaFile, String> pathColumn, artistColumn, titleColumn, albumColumn,
        trackNumberColumn, lengthColumn, descriptionColumn,
        ratingColumn, dateColumn, settingColumn, URLColumn, languageColumn,
        nowPlayingColumn, publishedByColumn, encodedByColumn, artworkURLColumn,
        trackIDColumn;

private static final String NEW_PLAYLIST = "New Playlist";
private static final String FRIEND_PLAYLIST = "Friend's Playlist";
private static final String LIBRARY = "Library";
private static ButtonCell bc;
ObservableList<String> observablePlayList;

@Override
public void initialize(URL url, ResourceBundle rb) {

    observablePlayList = FXCollections.observableArrayList();
    observablePlayList.add(NEW_PLAYLIST);
    observablePlayList.add(FRIEND_PLAYLIST);
    observablePlayList.add(LIBRARY);
    playList.setItems(observablePlayList);
    playList.setCellFactory(new Callback<ListView<String>, ListCell<String>>() {
        @Override
        public ListCell<String> call(ListView<String> list) {
            bc = new ButtonCell();
            return bc;
        }
    });

    playList.setEditable(true);
    playList.getSelectionModel().select(2);

    playList.setOnEditCommit(new EventHandler<ListView.EditEvent<String>>() {
        @Override
        public void handle(ListView.EditEvent<String> event) {

            System.out.println(event.getEventType());
            System.out.println(event.getNewValue());
        }
    });

    pathColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("path"));
    artistColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("artist"));
    titleColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("title"));
    albumColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("album"));
    trackNumberColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("trackNumber"));
    lengthColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("length"));
    descriptionColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("description"));
    ratingColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("rating"));
    dateColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("setting"));
    settingColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("url"));
    URLColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("language"));
    languageColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("nowPlaying"));
    nowPlayingColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("publisher"));
    publishedByColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("encodedBy"));
    encodedByColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("artworkURL"));
    artworkURLColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("artworkURL"));
    trackIDColumn.setCellValueFactory(new PropertyValueFactory<MediaFile, String>("trackID"));

    Callback<TableColumn<MediaFile, String>, TableCell<MediaFile, String>> cellFactory
            = new Callback<TableColumn<MediaFile, String>, TableCell<MediaFile, String>>() {
                @Override
                public TableCell call(TableColumn p) {
                    return new EditingCell();
                }
            };

    pathColumn.setCellFactory(cellFactory);
    artistColumn.setCellFactory(cellFactory);
    titleColumn.setCellFactory(cellFactory);
    albumColumn.setCellFactory(cellFactory);
    trackNumberColumn.setCellFactory(cellFactory);
    lengthColumn.setCellFactory(cellFactory);
    descriptionColumn.setCellFactory(cellFactory);
    ratingColumn.setCellFactory(cellFactory);
    dateColumn.setCellFactory(cellFactory);
    settingColumn.setCellFactory(cellFactory);
    URLColumn.setCellFactory(cellFactory);
    languageColumn.setCellFactory(cellFactory);
    nowPlayingColumn.setCellFactory(cellFactory);
    publishedByColumn.setCellFactory(cellFactory);
    encodedByColumn.setCellFactory(cellFactory);
    artworkURLColumn.setCellFactory(cellFactory);
    trackIDColumn.setCellFactory(cellFactory);

    Task loadDB = new Task() {

        @Override
        protected Object call() throws Exception {
            DBUtils.initDB();
            DBUtils.retrieveDB();
            return null;
        }
    };

    new Thread(loadDB).start();

    boolean dbFinishedLoading = false;
    while (dbFinishedLoading==false) {

        if (loadDB.isDone()) {
            System.out.println("done loading db!!!");
            System.out.println(DBUtils.getList());
            dbFinishedLoading = true;
            playListTableView.setItems(DBUtils.getList());
        }
    }

}

class EditingCell extends TableCell<MediaFile, String> {

    private TextField textField;

    public EditingCell() {
    }

    @Override
    public void startEdit() {
        if (!isEmpty()) {
            super.startEdit();
            createTextField();
            setText(null);
            setGraphic(textField);
            textField.selectAll();
        }
    }

    @Override
    public void cancelEdit() {
        super.cancelEdit();

        setText((String) getItem());
        setGraphic(null);
    }

    @Override
    public void updateItem(String item, boolean empty) {
        super.updateItem(item, empty);

        if (empty) {
            setText(null);
            setGraphic(null);
        } else {
            if (isEditing()) {
                if (textField != null) {
                    textField.setText(getString());
                }
                setText(null);
                setGraphic(textField);
            } else {
                setText(getString());
                setGraphic(null);
            }
        }
    }

    private void createTextField() {
        textField = new TextField(getString());
        textField.setMinWidth(this.getWidth() - this.getGraphicTextGap() * 2);
        textField.focusedProperty().addListener(new ChangeListener<Boolean>() {
            @Override
            public void changed(ObservableValue<? extends Boolean> arg0,
                    Boolean arg1, Boolean arg2) {
                if (!arg2) {
                    commitEdit(textField.getText());
                }
            }
        });
    }

    private String getString() {
        return getItem() == null ? "" : getItem().toString();
    }
}

class ButtonCell extends ListCell<String> {

    private TextField fieldForEditingCell;

    @Override
    public void startEdit() {

        if (!isEditable() || !getListView().isEditable()) {
            return;
        }

        super.startEdit();

        if (isEditing()) {
            fieldForEditingCell = new TextField("Enter your edit here");
            fieldForEditingCell.setOnKeyPressed(new EventHandler<KeyEvent>() {

                public void handle(KeyEvent keyPress) {
                    if (keyPress.getCode() == KeyCode.ENTER) {
                        String nameOfnewPlayList = fieldForEditingCell.getText();
                        observablePlayList.add(nameOfnewPlayList);
                        playList.setItems(null);
                        playList.setItems(observablePlayList);
                        playList.getSelectionModel().select(nameOfnewPlayList);

                    } else if (keyPress.getCode() == KeyCode.ESCAPE) {
                        cancelEdit();
                    }
                }
            });

            setGraphic(fieldForEditingCell);
            fieldForEditingCell.requestFocus();
            fieldForEditingCell.setEditable(true);

            fieldForEditingCell.focusedProperty().addListener(new ChangeListener<Boolean>() {
                @Override
                public void changed(ObservableValue<? extends Boolean> observable, Boolean oldValue, Boolean newValue) {
                    if (!newValue && fieldForEditingCell != null) {
                        commitEdit(fieldForEditingCell.getText());
                    }
                }
            });

            fieldForEditingCell.selectAll();

        }
    }

    @Override
    public void cancelEdit() {
        super.cancelEdit();
        ImageView addSymbol;
        addSymbol = ImageViewBuilder.create().image(new Image("/images/ic_add_grey600_15dp.png")).build();
        addSymbol.fitHeightProperty();
        setText(getItem());
        setGraphic(addSymbol);
    }

    @Override
    public void updateItem(String item, boolean empty) {
        super.updateItem(item, empty);

        if ((this.getIndex() == 0) || (this.getIndex() == 1)) {
            ImageView addSymbol;
            addSymbol = ImageViewBuilder.create().image(new Image("/images/ic_add_grey600_15dp.png")).build();
            addSymbol.fitHeightProperty();
            setText(item);
            setGraphic(addSymbol);

            getStyleClass().add("custom_list-cell");
            //setStyle("-fx-background-color: #F5F5F5");
            //setStyle("-fx-hover-color: yellow");
        } else {
            setText(item);
            setGraphic(null);
        }
    }

}
//begin TableViewCell

public void tableDraggedOVer(DragEvent event) {
    Dragboard db = event.getDragboard();
    if (db.hasFiles()) {
        event.acceptTransferModes(TransferMode.COPY);
    }
    event.consume();

}

public void dropCompleted(DragEvent event) {
    Dragboard db = event.getDragboard();
    boolean success = false;

    if (db.hasFiles()) {
        event.acceptTransferModes(TransferMode.COPY);
        DBUtils.addToPLaylistTableFromDragAndDrop(db.getFiles());
        success = true;
    }

    event.setDropCompleted(success);
    event.consume();
}

}