FileChangeWatcher: prevent exception when OpenedFile.FileName is null.

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@3000 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/Main/Base/Project/Src/Services/File/FileChangeWatcher.cs

@@ -100,6 +100,8 @@ namespace ICSharpCode.SharpDevelop
 				return;
 			
 			string fileName = file.FileName;
+			if (string.IsNullOrEmpty(fileName))
+				return;
 			if (FileUtility.IsUrl(fileName))
 				return;
 			if (!Path.IsPathRooted(fileName))