Error in Reading Excel using a referenced class

I am pretty new to c# and I've got a somehow strange problem. I've created a class in project1 that gets the path to an excel file, reads it, and returns the data in an array. The class code is like:

using System;
using Microsoft.Office.Interop.Excel;
using _Excel = Microsoft.Office.Interop.Excel;

namespace srcClasses.Data
{
    public class ExcelReaderCustom : IDisposable
    {
        public _Application Excel { get; set; }
        public Workbook Wb { get; set; }
        public Worksheet Ws { get; set; }

        public ExcelReaderCustom(string path)
        {
            Excel = new _Excel.Application();
            Wb = Excel.Workbooks.Open(@path);
            Ws = (_Excel.Worksheet)Wb.ActiveSheet; ;
        }

        public dynamic[,] ReturnData(bool ignoreFirstRow)
        {
            int offset;
            if (ignoreFirstRow){
                offset = 1;
            }
            else
            {
                offset = 0;
            }
            var data = new dynamic[Ws.UsedRange.Rows.Count-offset, Ws.UsedRange.Columns.Count];
            for (int i = offset; i < Ws.UsedRange.Rows.Count; i++)
            {
                for (int j = 0; j < Ws.UsedRange.Columns.Count; j++)
                {
                    data[i - offset, j] = Ws.Cells[i + 1, j + 1].value;
                }
            }
            return data;
        }

        public void Dispose()
        {
            Wb.Close();
            Excel.Quit();
        }
    }
}

The way I use this class is like:

dynamic[,] filterN;
var path = //Path to a sample excel;
using (var filterExcel = new ExcelReaderCustom(path))
{
    filterN = filterExcel.ReturnData(ignoreFirstRow: true);
}

Everything works fine when I'm using the class in the same project. However, I've add a reference to project1 in another project named project2 and when I try to use this class in the second project with the very same code I get an System.NullReferenceException when calling objects from "Ws" like in the following line:

var data = new dynamic[Ws.UsedRange.Rows.Count-offset,aWs.UsedRange.Columns.Count];