Resolves: tdf#124142 don't deref nullptr

Change-Id: Ie14600e9f9a1e1c4e99c7a872f5d677453481888
Reviewed-on: https://gerrit.libreoffice.org/69666
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit ac7ce7a6)
Reviewed-on: https://gerrit.libreoffice.org/69694Reviewed-by: Xisco Faulí <xiscofauli@libreoffice.org>

sw/source/core/unocore/unostyle.cxx

@@ -193,7 +193,8 @@ namespace sw
             , m_pBasePool(pDocShell->GetStyleSheetPool())
             , m_pDocShell(pDocShell)
         {
-            StartListening(*m_pBasePool);
+            if (m_pBasePool) //tdf#124142 html docs can have no styles
+                StartListening(*m_pBasePool);
         }
 
         //XIndexAccess