Program: 77988
Project: 77988
Buggy source:
private void prepareHBox(at.ac.tuwien.sepm.ws16.qse01.entities.Image img){
  HBox hBox=new HBox();
  hBox.setSpacing(120);
  hBox.setVisible(false);
  hBox.setStyle("String_Node_Str");
  ImageView fullscreen=new ImageView(new Image("String_Node_Str"));
  fullscreen.setFitHeight(30);
  fullscreen.setFitWidth(30);
  fullscreen.setOnMouseClicked(mouseEvent -> {
    ImageView imageView=(ImageView)((VBox)(((ImageView)mouseEvent.getSource()).getParent().getParent())).getChildren().get(0);
    LOGGER.debug("String_Node_Str" + imageView.getId());
    windowManager.showFullscreenImage(Integer.parseInt(imageView.getId()));
  }
);
  ImageView delete=new ImageView(getClass().getResource("String_Node_Str").toExternalForm());
  delete.setFitHeight(30);
  delete.setFitWidth(30);
  delete.setOnMouseClicked((  MouseEvent mouseEvent) -> {
    LOGGER.debug("String_Node_Str");
    Alert alert=new Alert(Alert.AlertType.CONFIRMATION);
    alert.setHeaderText("String_Node_Str");
    alert.setContentText("String_Node_Str");
    alert.initModality(Modality.WINDOW_MODAL);
    alert.initOwner(this.stage);
    Optional<ButtonType> result=alert.showAndWait();
    if (result.isPresent() && result.get() == ButtonType.OK) {
      ImageView imageView=(ImageView)((VBox)(((ImageView)mouseEvent.getSource()).getParent().getParent())).getChildren().get(0);
      LOGGER.debug("String_Node_Str" + imageView.getId());
      try {
        imageService.delete(Integer.parseInt(imageView.getId()));
        tile.getChildren().remove(imageView.getParent());
      }
 catch (      ServiceException e) {
        LOGGER.debug("String_Node_Str" + e.getMessage());
      }
    }
  }
);
  hBox.getChildren().addAll(fullscreen,delete);
  ImageView imageView=null;
  try {
    if (new File(img.getImagepath()).isFile()) {
      imageView=createImageView(new File(img.getImagepath()));
    }
 else     if (new File(System.getProperty("String_Node_Str") + "String_Node_Str" + img.getImagepath()).isFile()) {
      img.setImagepath(System.getProperty("String_Node_Str") + "String_Node_Str" + img.getImagepath());
      imageService.update(img);
      imageView=createImageView(new File(img.getImagepath()));
    }
 else {
      LOGGER.debug("String_Node_Str" + img.toString());
      imageService.delete(img.getImageID());
    }
    if (imageView != null) {
      VBox vBox=new VBox();
      LOGGER.debug("String_Node_Str" + img.getImageID());
      imageView.setId(String.valueOf(img.getImageID()));
      imageView.setUserData(img.getImagepath());
      vBox.getChildren().addAll(imageView,hBox);
      tile.getChildren().add(vBox);
    }
  }
 catch (  Exception e) {
    LOGGER.debug("String_Node_Str" + e.getMessage());
  }
}

Fixed version (ground truth):
private void prepareHBox(at.ac.tuwien.sepm.ws16.qse01.entities.Image img){
  HBox hBox=new HBox();
  hBox.setSpacing(120);
  hBox.setVisible(false);
  hBox.setStyle("String_Node_Str");
  ImageView fullscreen=new ImageView(new Image("String_Node_Str"));
  fullscreen.setFitHeight(30);
  fullscreen.setFitWidth(30);
  fullscreen.setOnMouseClicked(mouseEvent -> {
    ImageView imageView=(ImageView)((VBox)(((ImageView)mouseEvent.getSource()).getParent().getParent())).getChildren().get(0);
    LOGGER.debug("String_Node_Str" + imageView.getId());
    windowManager.showFullscreenImage(Integer.parseInt(imageView.getId()));
  }
);
  ImageView delete=new ImageView(getClass().getResource("String_Node_Str").toExternalForm());
  delete.setFitHeight(30);
  delete.setFitWidth(30);
  delete.setOnMouseClicked((  MouseEvent mouseEvent) -> {
    LOGGER.debug("String_Node_Str");
    Alert alert=new Alert(Alert.AlertType.CONFIRMATION);
    alert.setHeaderText("String_Node_Str");
    alert.setContentText("String_Node_Str");
    alert.initModality(Modality.WINDOW_MODAL);
    alert.initOwner(this.stage);
    Optional<ButtonType> result=alert.showAndWait();
    if (result.isPresent() && result.get() == ButtonType.OK) {
      ImageView imageView=(ImageView)((VBox)(((ImageView)mouseEvent.getSource()).getParent().getParent())).getChildren().get(0);
      LOGGER.debug("String_Node_Str" + imageView.getId());
      try {
        imageService.delete(Integer.parseInt(imageView.getId()));
        tile.getChildren().remove(imageView.getParent());
      }
 catch (      ServiceException e) {
        LOGGER.debug("String_Node_Str" + e.getMessage());
      }
    }
  }
);
  hBox.getChildren().addAll(fullscreen,delete);
  ImageView imageView=null;
  try {
    if (new File(img.getImagepath()).isFile()) {
      imageView=createImageView(new File(img.getImagepath()));
    }
 else {
      LOGGER.debug("String_Node_Str" + img.toString());
      imageService.delete(img.getImageID());
    }
    if (imageView != null) {
      VBox vBox=new VBox();
      LOGGER.debug("String_Node_Str" + img.getImageID());
      imageView.setId(String.valueOf(img.getImageID()));
      imageView.setUserData(img.getImagepath());
      vBox.getChildren().addAll(imageView,hBox);
      tile.getChildren().add(vBox);
    }
  }
 catch (  Exception e) {
    LOGGER.debug("String_Node_Str" + e.getMessage());
  }
}