Fixed Thread.Callstack yielding null.

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@671 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/AddIns/Misc/Debugger/Debugger.Core/Project/Src/Threads/Thread.cs

@@ -219,7 +219,10 @@ namespace DebuggerLibrary
 						} catch (COMException) {
 							// TODO
 						};
-						yield return function;
+						
+						if (function != null) {
+							yield return function;
+						}
 					}
 				}
 			} // get