GridPane.getColumnIndex() returning null - javafx

Question

Trying to add children to a GridPane with row and column indices. They show up in the gui, but using GridPane.getColumnIndex(node) always returns null.
GridPane grid = new GridPane();
for (int i = 0; i < 20; i++) {
for (int j = 0; j < 20; j++) {
grid.setGridLinesVisible(true);
Button myBtn = new Button("foo");
GridPane.setColumnIndex(myButton,i);
GridPane.setRowIndex(myButton,j);
grid.getChildren().add(myButton);
}
When I try to use this algorithm I got from the answer on javafx GridPane retrive specific Cell content, the program crashes with a NullPointerException.
private Node getNodeFromGridPane(GridPane gridPane, int col, int row)
{
for (Node node : gridPane.getChildren()) {
if (GridPane.getColumnIndex(node) == col && GridPane.getRowIndex(node) == row) {
return node;
}
}
return null;
}
Why are GridPane.getRowIndex() and GridPane.getColumnIndex() returning null even after calling setColumnIndex() and setRowIndex()?

Answer 1

So it seems I got it to work if you run it you will see that all the buttons are red white or blue this is because they were found
import javafx.application.Application;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.GridPane;
import javafx.stage.Stage;
public class Main extends Application {
#Override
public void start(Stage primaryStage) {
GridPane grid = new GridPane();
grid.setGridLinesVisible(true);
for (int i = 0; i < 20; i++) {
for (int j = 0; j < 20; j++) {
grid.add(new Button("foo"),i,j);
// Button myBtn = new Button("foo");
// GridPane.setColumnIndex(myBtn, i);
// GridPane.setRowIndex(myBtn, j);
// grid.getChildren().add(myBtn);
}
}
Scene scene = new Scene(grid);
Stage stage = new Stage();
stage.setScene(scene);
stage.show();
String[] colors = {"-fx-background-color: red;", "-fx-background-color: blue;", "-fx-background-color: white;"};
for (int i = 0; i < 20; i++) {
for (int j = 0; j < 20; j++) {
Node node = getNodeFromGridPane(grid,i,j);
if(node instanceof Button)
node.setStyle(colors[(int) (Math.random() * 3)]);
}
}
}
private Node getNodeFromGridPane(GridPane gridPane, int col, int row) {
for (Node node : gridPane.getChildren())
if (GridPane.getColumnIndex(node) != null
&& GridPane.getColumnIndex(node) != null
&& GridPane.getRowIndex(node) == row
&& GridPane.getColumnIndex(node) == col)
return node;
return null;
}
public static void main(String[] args) { launch(args); }
}