Object reference not set in xmlnode.selectsinglenode()

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C#

.NET4.5

I am trying to get values from specific fields from a string which is XML format.

My Code:

static void Main(string[] args)
        {
            String str;
            str = "<layer> <auto_delta>null</auto_delta> <frame-number>5236</frame-number> <nb_frames>45001</nb_frames> <frames-left>39765</frames-left> <frame-age>160</frame-age> <producer>  <type>ffmpeg</type>  <filename>media\\JTV.AVI</filename>  <width>720</width>  <height>576</height>  <progressive>true</progressive>  <fps>25</fps> <loop>false</loop> <frame-number>5237</frame-number> <nb-frames>45001</nb-frames>  <file-frame-number>0</file-frame-number>  <file-nb-frames>45001</file-nb-frames> </producer> <background> <producer> <type>empty-producer</type> </producer> </background> <index>0</index></layer>";
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.LoadXml(str);
            string frame_path = "layer/producer";
            var nodes_frame = xmlDoc.SelectNodes(frame_path);

            try
            {
                foreach (XmlNode childrenNode_frame in nodes_frame)
                {
                    Console.WriteLine("Frames: " + childrenNode_frame.SelectSingleNode("//file-nb-frames").Value.ToString() + " Frame Rate: " + childrenNode_frame.SelectSingleNode("//fps").Value.ToString());
                }
            }
            catch (Exception err)
            {
                Console.WriteLine(err.ToString());
            }

            Console.Read();
        }

Now getting Exception Object re reference not set in

Console.WriteLine("Frames: " + childrenNode_frame.SelectSingleNode("//file-nb-frames").Value.ToString() + " Frame Rate: " + childrenNode_frame.SelectSingleNode("//fps").Value.ToString());

this line.

Please help me. Thanks in advance.

What I have tried:

I have used Load() method. Than I have got another exception as like this: "Additional information: Illegal characters in path."