<@U2CKX5M3M> - when you call the stackpane{} build...

# tornadofx

[gtnarg]

@slicky - when you call the stackpane{} builder function - by virtue of it being a builder function it implicity adds the new StackPane node to the GridPane. When you subsequently call add( cell, col, row ), the cell is already a child node of the gridpane - hence the exception.