Why do I get a "duplicate children" exception when creating GridPane children?

Issue

I am trying to dynamically create a GridPane view in JavaFX. The issue is when trying to add the children to the pane I get a "duplicate children" exception.

Here is how I adding the children to the pane:

    GridPane gridView = new GridPane();
    for(int i = 0; i < mobster.getActions().size(); i++) {
        MobsterVisualWrapper actionWrapper = new MobsterVisualWrapper(mobster.getActionList().get(i));

        gridView.add(actionWrapper.getNode(), 0, i);
        gridView.add(actionWrapper.getLabel(), 1, i);
    }
    
    Platform.runLater(() -> {
        queuedActionsBorderPane.setCenter(gridView);
    });



public static class MobsterVisualWrapper{
        private final Label actionLabel;
        private final ProgressBar progressBar = new ProgressBar(-1.0f);
        private final ImageView imageView = new ImageView();
        private Node node;
        
        public MobsterVisualWrapper(AbstractAction action) {
            actionLabel = new Label(action.getName());
            actionLabel.setDisable(true);
            node = new Label("-");
            action.setVisualWrapper(this);
        }
        
        public void setRunning(boolean running) {
            if(running) {
                setNode(progressBar);
            } else {
                setNode(new ImageView());
            }
        }
        
        public void setFailed() {
            imageView.setId("actionViewFailed");
            setNode(imageView);
        }
        
        public void setComplete() {
            imageView.setId("actionViewComplete");
            setNode(imageView);
        }
        
        /**
         * Sets the node that will show the current status of the action
         * @param node the node to show the status
         */
        private void setNode(Node node) {
            this.node = node;
        }
        
        /**
         * @return the node to show the status of the action
         */
        private Node getNode() {
            return node;
        }
        
        public Label getLabel() {
            return actionLabel;
        }
    }

I am trying to add a new row each loop iteration with two columns.

Solution

This code actually does work and no longer throws the exception. I think before I was setting GridPane.setConstraints along with the add method which may have caused some issues. I thought the issue was still persisting despite removing this but I guess that isn't the case. Thanks to everyone who tried to help.

Answered By - Adrian Elder