System.NullReferenceException: Object reference not set to an instance of an object.

private void button1_Click(object sender, System.EventArgs e)
{
	object oMissing = System.Reflection.Missing.Value;
	object oEndOfDoc = "\\endofdoc"; /* \endofdoc is a predefined bookmark */ 

	//Start Word and create a new document.
	Word._Application oWord;
	Word._Document oDoc;
	oWord = new Word.Application();
	oWord.Visible = true;
	oDoc = oWord.Documents.Add(ref oMissing, ref oMissing,
		ref oMissing, ref oMissing);

	//Insert a paragraph at the beginning of the document.
	Word.Paragraph oPara1;
	oPara1 = oDoc.Content.Paragraphs.Add(ref oMissing);
	oPara1.Range.Text = "Heading 1";
	oPara1.Range.Font.Bold = 1;
	oPara1.Format.SpaceAfter = 24;    //24 pt spacing after paragraph.
	oPara1.Range.InsertParagraphAfter();

	//Insert a paragraph at the end of the document.
	Word.Paragraph oPara2;
	object oRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
	oPara2 = oDoc.Content.Paragraphs.Add(ref oRng);
	oPara2.Range.Text = "Heading 2";
	oPara2.Format.SpaceAfter = 6;
	oPara2.Range.InsertParagraphAfter();

	//Insert another paragraph.
	Word.Paragraph oPara3;
	oRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
	oPara3 = oDoc.Content.Paragraphs.Add(ref oRng);
	oPara3.Range.Text = "This is a sentence of normal text. Now here is a table:";
	oPara3.Range.Font.Bold = 0;
	oPara3.Format.SpaceAfter = 24;
	oPara3.Range.InsertParagraphAfter();

	//Insert a 3 x 5 table, fill it with data, and make the first row
	//bold and italic.
	Word.Table oTable;
	Word.Range wrdRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
	oTable = oDoc.Tables.Add(wrdRng, 3, 5, ref oMissing, ref oMissing);
	oTable.Range.ParagraphFormat.SpaceAfter = 6;
	int r, c;
	string strText;
	for(r = 1; r <= 3; r++)
		for(c = 1; c <= 5; c++)
		{
			strText = "r" + r + "c" + c;
			oTable.Cell(r, c).Range.Text = strText;
		}
	oTable.Rows[1].Range.Font.Bold = 1;
	oTable.Rows[1].Range.Font.Italic = 1;

	//Add some text after the table.
	Word.Paragraph oPara4;
	oRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
	oPara4 = oDoc.Content.Paragraphs.Add(ref oRng);
	oPara4.Range.InsertParagraphBefore();
	oPara4.Range.Text = "And here's another table:";
	oPara4.Format.SpaceAfter = 24;
	oPara4.Range.InsertParagraphAfter();

	//Insert a 5 x 2 table, fill it with data, and change the column widths.
	wrdRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
	oTable = oDoc.Tables.Add(wrdRng, 5, 2, ref oMissing, ref oMissing);
	oTable.Range.ParagraphFormat.SpaceAfter = 6;
	for(r = 1; r <= 5; r++)
		for(c = 1; c <= 2; c++)
		{
			strText = "r" + r + "c" + c;
			oTable.Cell(r, c).Range.Text = strText;
		}
	oTable.Columns[1].Width = oWord.InchesToPoints(2); //Change width of columns 1 & 2
	oTable.Columns[2].Width = oWord.InchesToPoints(3);

	//Keep inserting text. When you get to 7 inches from top of the
	//document, insert a hard page break.
	object oPos;
	double dPos = oWord.InchesToPoints(7);
	oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range.InsertParagraphAfter();
	do
	{
		wrdRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
		wrdRng.ParagraphFormat.SpaceAfter = 6;
		wrdRng.InsertAfter("A line of text");
		wrdRng.InsertParagraphAfter();
		oPos = wrdRng.get_Information
                       (Word.WdInformation.wdVerticalPositionRelativeToPage);
	}
	while(dPos >= Convert.ToDouble(oPos));
	object oCollapseEnd = Word.WdCollapseDirection.wdCollapseEnd;
	object oPageBreak = Word.WdBreakType.wdPageBreak;
	wrdRng.Collapse(ref oCollapseEnd);
	wrdRng.InsertBreak(ref oPageBreak);
	wrdRng.Collapse(ref oCollapseEnd);
	wrdRng.InsertAfter("We're now on page 2. Here's my chart:");
	wrdRng.InsertParagraphAfter();

	//Insert a chart.
	Word.InlineShape oShape;
	object oClassType = "MSGraph.Chart.8";
	wrdRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
	oShape = wrdRng.InlineShapes.AddOLEObject(ref oClassType, ref oMissing, 
		ref oMissing, ref oMissing, ref oMissing,
		ref oMissing, ref oMissing, ref oMissing);

	//Demonstrate use of late bound oChart and oChartApp objects to
	//manipulate the chart object with MSGraph.
	object oChart;
	object oChartApp;
	oChart = oShape.OLEFormat.Object;
	oChartApp = oChart.GetType().InvokeMember("Application",
		BindingFlags.GetProperty, null, oChart, null);

	//Change the chart type to Line.
	object[] Parameters = new Object[1];
	Parameters[0] = 4; //xlLine = 4
	oChart.GetType().InvokeMember("ChartType", BindingFlags.SetProperty,
		null, oChart, Parameters);

	//Update the chart image and quit MSGraph.
	oChartApp.GetType().InvokeMember("Update",
		BindingFlags.InvokeMethod, null, oChartApp, null);
	oChartApp.GetType().InvokeMember("Quit",
		BindingFlags.InvokeMethod, null, oChartApp, null);
	//... If desired, you can proceed from here using the Microsoft Graph 
	//Object model on the oChart and oChartApp objects to make additional
	//changes to the chart.

	//Set the width of the chart.
	oShape.Width = oWord.InchesToPoints(6.25f);
	oShape.Height = oWord.InchesToPoints(3.57f);

	//Add text after the chart.
	wrdRng = oDoc.Bookmarks.get_Item(ref oEndOfDoc).Range;
	wrdRng.InsertParagraphAfter();
	wrdRng.InsertAfter("THE END.");

	//Close this form.
	this.Close();
}