sw_redlinehide_4b: xmloff: destroy TextImportHelper before Model

nullpointer in ~XMLRedlineImportHelper in
SwUiWriterTest::testThreadedException()

Change-Id: Ic4a79ceffa831aced4db47836333a15a13773887

xmloff/source/core/xmlimp.cxx

@@ -431,6 +431,7 @@ void SvXMLImport::cleanup() throw ()
             pStylesContext->Clear();
         maContexts.pop();
     }
+    mxTextImport.clear(); // XMLRedlineImportHelper needs model
     DisposingModel();
 }