Word Export LT-21942: Fixed more Export failures

When testing with Steve’s Project additional failures were found:
1. Open image files with read access. The same image can be used
by multiple entries. We were trying to open the same file with
write access from different threads.
2. Use FirstOrDefault() instead of First().

Change-Id: I1ac9ef7f21324165c0d1494f66aa80cdc58d8fb1

Src/xWorks/LcmWordGenerator.cs

@@ -1951,7 +1951,7 @@ public static string AddImagePartToPackage(WordprocessingDocument doc, string im
 		{
 			MainDocumentPart mainPart = doc.MainDocumentPart;
 			ImagePart imagePart = mainPart.AddImagePart(imageType);
-			using (FileStream stream = new FileStream(imagePath, FileMode.Open))
+			using (FileStream stream = new FileStream(imagePath, FileMode.Open, FileAccess.Read))
 			{
 				imagePart.FeedData(stream);
 			}
@@ -2418,7 +2418,7 @@ private void AddBulletAndNumberingData(IFragment elementContent, ConfigurableDic
 						{
 							if (elem is Paragraph)
 							{
-								var paraProps = elem.Elements<ParagraphProperties>().First();
+								var paraProps = elem.Elements<ParagraphProperties>().FirstOrDefault();
 								if (paraProps != null)
 								{
 									// Only add the uniqueId to paragraphs with the correct style.  There could