Getting the path of linked (smart) object / PS Bug?

The code you found works for linked smart object layers it will through an error if the object is not a linked file. You can use a try catch to note the and ignore none linked smart object layers.

function processSmartObj(obj) {

  var localFilePath = "";

  var ref = new ActionReference();  

    ref.putIdentifier(charIDToTypeID('Lyr '), obj.id );  

    var desc = executeActionGet(ref);  

    var smObj = desc.getObjectValue(stringIDToTypeID('smartObject'));  

  try {

  var localFilePath = smObj.getPath(stringIDToTypeID('link')); 

  alert("Layer " + obj.name + ' linked file is"' +  localFilePath + '"');

  }

  catch(e) {}

  return localFilePath;

}

You could put some boilerplate around the process to structure the code an make it  more readable  for example process all layer in a document may look something like this.

/* ==========================================================
// 2017  John J. McAssey (JJMack) 
// ======================================================= */
// This script is supplied as is. It is provided as freeware. 
// The author accepts no liability for any problems arising from its use.
// enable double-clicking from Mac Finder or Windows Explorer
#target photoshop // this command only works in Photoshop CS2 and higher
// bring application forward for double-click events
app.bringToFront();
// ensure at least one document open
if (!documents.length) alert('There are no documents open.', 'No Document');
else {
  // declare Global variables
  //main(); // at least one document exists proceed
    app.activeDocument.suspendHistory('Some Process Name','main()');
}
///////////////////////////////////////////////////////////////////////////////
//                            main function                                  //
///////////////////////////////////////////////////////////////////////////////
function main() {
  // declare local variables
  var orig_ruler_units = app.preferences.rulerUnits;
  var orig_type_units = app.preferences.typeUnits;
  var orig_display_dialogs = app.displayDialogs;
  app.preferences.rulerUnits = Units.PIXELS;   // Set the ruler units to PIXELS
  app.preferences.typeUnits = TypeUnits.POINTS;   // Set Type units to POINTS
  app.displayDialogs = DialogModes.NO;     // Set Dialogs off
  try { code(); }
  // display error message if something goes wrong
  catch(e) { alert(e + ': on line ' + e.line, 'Script Error', true); }
  app.displayDialogs = orig_display_dialogs;   // Reset display dialogs 
  app.preferences.typeUnits  = orig_type_units; // Reset ruler units to original settings 
  app.preferences.rulerUnits = orig_ruler_units; // Reset units to original settings
}
///////////////////////////////////////////////////////////////////////////////
//                           main function end                               //
///////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////////
// The real code is embedded into this function so that at any point it can return //
// to the main line function to let it restore users edit environment and end      //
/////////////////////////////////////////////////////////////////////////////////////
function code() {
  processArtLayers(activeDocument)  
}
function processArtLayers(obj) {  
    for( var i = obj.artLayers.length-1; 0 <= i; i--) {processLayers(obj.artLayers);}  
    for( var i = obj.layerSets.length-1; 0 <= i; i--) {processArtLayers(obj.layerSets); } // Process Layer Set Layers  
} 
function processLayers(layer) { 
  switch (layer.kind){
  case LayerKind.SMARTOBJECT : processSmartObj(layer); break;
  default : break; // none process layer types catch all
  }
} 
//////////////////////////////////////////////////////////////////////////////////
// Layer Type Functions //
//////////////////////////////////////////////////////////////////////////////////
function processSmartObj(obj) { 
  var localFilePath = "";
  var ref = new ActionReference();    
    ref.putIdentifier(charIDToTypeID('Lyr '), obj.id );    
    var desc = executeActionGet(ref);    
    var smObj = desc.getObjectValue(stringIDToTypeID('smartObject'));    
  try {
  var localFilePath = smObj.getPath(stringIDToTypeID('link'));   
  alert("Layer " + obj.name + ' linked file is"' +  localFilePath + '"');
  }
  catch(e) {}
  return localFilePath;
}