tdf#108402: avoid crash if filter doesn't match

See bt https://bugs.documentfoundation.org/attachment.cgi?id=134006
+ display warning

Change-Id: I38496364c0965015ea54f753f3d3550f317d88d8
Reviewed-on: https://gerrit.libreoffice.org/60413
Tested-by: Jenkins
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

sw/source/filter/xml/xmlexp.cxx

@@ -124,6 +124,8 @@ ErrCode SwXMLExport::exportDoc( enum XMLTokenEnum eClass )
     }
 
     SwDoc *pDoc = getDoc();
+    if (!pDoc)
+        return ERR_SWG_WRITE_ERROR;
 
     // Make sure the layout is available to have more stability in the output
     // markup.
@@ -514,6 +516,12 @@ SwDoc* SwXMLExport::getDoc()
     if( m_pDoc != nullptr )
         return m_pDoc;
     Reference < XTextDocument > xTextDoc( GetModel(), UNO_QUERY );
+    if (!xTextDoc)
+    {
+        SAL_WARN("sw.filter", "Problem of mismatching filter for export.");
+        return nullptr;
+    }
+
     Reference < XText > xText = xTextDoc->getText();
     Reference<XUnoTunnel> xTextTunnel( xText, UNO_QUERY);
     assert( xTextTunnel.is());