Don't attempt to get surrounding text of error when we don't have access to script string anymore.

Upsilon/Diagnostics.cs

@@ -33,7 +33,9 @@ namespace Upsilon
         {
             if (ThrowsOnError)
             {
-                var text = ScriptString.GetSpan(location);
+                string text = string.Empty;
+                if (ScriptString != null)
+                    text = ScriptString.GetSpan(location);
                 throw new ParseException(FileName, message, location.StartLine, location.StartPosition, text);
             }
             Log(DiagnosticLevel.Error, message, location);