fixed exception when referenced project that was thrown when a referenced project wasn't part of the open project

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@283 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/Main/Base/Project/Src/Services/ParserService/ProjectContentRegistry.cs

@@ -69,6 +69,10 @@ namespace ICSharpCode.Core
 		public static IProjectContent GetProjectContentForReference(ReferenceProjectItem item)
 		{
 			if (item is ProjectReferenceProjectItem) {
+                if (((ProjectReferenceProjectItem)item).ReferencedProject == null)
+                {
+                    return null;
+                }
 				return ParserService.GetProjectContent(((ProjectReferenceProjectItem)item).ReferencedProject);
 			}
 			lock (contents) {