ofz#5834 Null-dereference

Change-Id: If52162286c949bd8c4f283ca8669fb1a99a61b0c
Reviewed-on: https://gerrit.libreoffice.org/48897Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sc/source/core/tool/formulalogger.cxx

@@ -339,7 +339,7 @@ FormulaLogger::GroupScope FormulaLogger::enterGroup(
 {
     // Get the file name if available.
     const SfxObjectShell* pShell = rDoc.GetDocumentShell();
-    const SfxMedium* pMedium = pShell->GetMedium();
+    const SfxMedium* pMedium = pShell ? pShell->GetMedium() : nullptr;
     OUString aName;
     if (pMedium)
         aName = pMedium->GetURLObject().GetLastName();