Problem while getting properties out of a Assembly

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
using System.Reflection;
using SolidEdgeFramework;
using SolidEdgeFileProperties;
using SolidEdgeAssembly;
using SolidEdgePart;
using SolidEdgeFrameworkSupport;

namespace Stuecklisten_Tool_V_1_00
{
    class Program
    {
        static void Main(string[] args)
        {
            SolidEdgeFramework.Application application=null;
            SolidEdgeFramework.Documents documents = null;
            SolidEdgeAssembly.AssemblyDocument assembly = null;
            SolidEdgeAssembly.Occurrences occurrences = null;
            //SolidEdgeAssembly.Occurrence objOccurrence =null;
            SolidEdgePart.PartDocument objSEDocument =null;
            //SolidEdgeFileProperties.PropertySets propertySets =null;
            //SolidEdgeFileProperties.Properties properties= null;
            //SolidEdgeFileProperties.Property property =null;


            // define Bom-Array
            string[][] BOM;

            // counter vars
            int i;  //BOM-row-number
            int j;  //fill assembly information in array

                       

            // Connect to a running instance of Solid Edge
            application = (SolidEdgeFramework.Application)
                Marshal.GetActiveObject("SolidEdge.Application");

            // Ausgabe der Verbindungsbestätgung in der Console.
            Console.WriteLine("Mit SolideEdge verbunden!");
            Console.ReadLine();

            // Get a Reference to the documents collection
            documents = application.Documents;

            // Get a Reference to the ActiveDocument 
            // and show the name
            
            assembly= (SolidEdgeAssembly.AssemblyDocument)
                 application.ActiveDocument;

            // Get reference to the occurrences collection
            occurrences = assembly.Occurrences;

            // count all occurrences
            // define size of Bom-array

            i = occurrences.Count;
            BOM = new string[i][];

            //fill Bom with assemblyinformation of all occurences
            for (j = 1; j <= i; j++)
            {
                BOM[j-1] = new string[6];
                //write occurence name in array --> ROW 1
                BOM[j-1][1] = occurrences.Item(j).Name;
                //write occurence number in array --> ROW 2
                BOM[j-1][2] = Convert.ToString(occurrences.Item(j).Quantity);
                // write path in array --> ROW 3
                BOM[j-1][3] = occurrences.Item(j).OccurrenceFileName;
                // write material in array --> ROW 4
                objSEDocument = (SolidEdgePart.PartDocument)
                    occurrences.Item(j).OccurrenceDocument;

                BOM[j-1][4] = objSEDocument.Properties("MechanicalModeling").Item("Material").value;
            }
        }
    }