Removed use of ?. because it doesn't compile in VS2013
Removed use of ?. because it doesn't compile in VS2013

--- a/KerbalEngineer.Unity/CanvasGroupFader.cs
+++ b/KerbalEngineer.Unity/CanvasGroupFader.cs
@@ -101,7 +101,8 @@
             }
 
             print(m_CanvasGroup.alpha);
-            callback?.Invoke();
+            if (callback != null)
+				callback.Invoke();
 
             m_FadeCoroutine = null;
         }