Small fix for not connected and converting

Author: rappen

FetchXmlBuilder/DockControls/TreeBuilderControl.cs

@@ -262,6 +262,10 @@ internal string GetFetchString(bool format, bool validate)
         internal FetchType GetFetchType()
         {
             var fetchstr = GetFetchString(false, false);
+            if (string.IsNullOrEmpty(fetchstr))
+            {
+                return null;
+            }
             var serializer = new XmlSerializer(typeof(FetchType));
             object result;
             using (TextReader reader = new StringReader(fetchstr))