Scripting Rename layer with Artboard Size Prefix

Looking for a way to rename a selected layer using the artboard size that the layer is within.
When using Generator, I'm hoping to be able to add that as a folder name to the start of the file name.
I allmost have this script working, but it grabs the first layer size, not the selected size.

var layer = app.activeDocument.layers[0];

Will grab the first layer, not selected layer

var layer = app.activeDocument.activeLayer;
Gives error:
Error 21: undefined is not an object.
Line: 12
-> var width = (ab[2] - ab[0]);

Based on this from @Stephen Marsh 
https://community.adobe.com/t5/photoshop-ecosystem-discussions/automatically-change-artboard-name-to-artboard-dimensions-in-pixels/m-p/13300750#M681172

// Retrieve the first layer of the active document
var  layer = app.activeDocument.layers[0];
//var  layer = app.activeDocument.activeLayer;

//alert(layer)


// Calculate the bounds of the artboard containing the selected layer
var ab = artboard_rectangle(layer);

// Calculate half of the artboard width and height
var width = (ab[2] - ab[0]);
var height = (ab[3] - ab[1]);

// Construct a new name for the layer using half of the artboard's width and height
var newName = width + "x" + height;

// Rename the selected layer with the new name
layer.name = newName;

// Check if there is an active document
if (app.activeDocument) {
    // Get the currently selected layer
    var selectedLayer = app.activeDocument.activeLayer;

    if (selectedLayer) {
        // Prefix to be added
        var prefix = width + "x" + height + '/';

        // Check if the layer name is not empty
        if (selectedLayer.name !== "") {
            // Add the prefix to the layer name
            selectedLayer.name = prefix + selectedLayer.name;
        } else {
            alert("Selected layer has no name.");
        }
    } else {
        alert("No layer is selected.");
    }
} else {
    alert("No active document found.");
}


// Function to calculate the bounds of an artboard
function artboard_rectangle(layer) {
    try {        
        var r = new ActionReference();    
        r.putProperty(stringIDToTypeID("property"), stringIDToTypeID("artboard"));

        if (layer) r.putIdentifier(stringIDToTypeID("layer"), layer.id);
        else       r.putEnumerated(stringIDToTypeID("layer"), stringIDToTypeID("ordinal"), stringIDToTypeID("targetEnum"));

        var d = executeActionGet(r).getObjectValue(stringIDToTypeID("artboard")).getObjectValue(stringIDToTypeID("artboardRect"));
        var bounds = new Array();

        bounds[0] = d.getUnitDoubleValue(stringIDToTypeID("top"));
        bounds[1] = d.getUnitDoubleValue(stringIDToTypeID("left"));
        bounds[2] = d.getUnitDoubleValue(stringIDToTypeID("right"));
        bounds[3] = d.getUnitDoubleValue(stringIDToTypeID("bottom"));

        return bounds;
    } catch(e) { 
        alert(e); 
    }
}