Ah.. you're not trying to capture a remote button press, but rather a poster you've added to the grid. In that case, I think you'd need to set the value of a field on the DetailsScreen that the Scene is observing. Perhaps a field called "close" (similiar to how dialogs work). The scene will observe that field and change the visibility when it's value changes.
The code above won't work, because your details screen doesn't have a reference to m.gridScreen or m.detailsScreen. Those are defined in the scene script.
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