How to get event target?

Action Manager does not have direct access to objects in the Object Model. It has it's own classes and object. Sometimes there can be a more direct access than by name. If the event descriptor had the layer index or ID you could use that instead of the name.

Sorry, when I threw this together for you I only tested hidding layers that were not the activeLayer. I don't think the reason the alert is empty is because the top layer was hidden. I think it's because the activeLayer was hidden so the actionReference is different.

Try this( seems to work for me with both )

try {
    if (arguments.length >= 2) {
    var desc = arguments[0];
    var event = arguments[1];
    // make sure it's the hide event
    if (event == charIDToTypeID('Hd  ')) {
        // get the list of what was hidden
        var list = desc.getList(charIDToTypeID('null'));
        // get the actionReferences from the list
        var ref = list.getReference(0);
        var psClass = ref.getDesiredClass();
        // make sure it was a layer that was hidded
        if(psClass == charIDToTypeID('Lyr ')){
            // check to see what is in the reference
            var dataEnum = ref.getForm();
            // should either be an enum if activeLayer
            if(dataEnum == ReferenceFormType.ENUMERATED) {
                    alert(app.activeDocument.activeLayer.name);
            }else{// or the layer name
                alert(ref.getName());
            }
        }
    }
  }
} catch (e) {
  alert( "Error: " + e + ":" + e.line );
}