Guard against NULL m_fileHistory in wxDocManager::OnMRUFile().

Don't crash if wxDocManager was created with "initialize = false" ctor
argument in which case m_fileHistory remains false.

Closes #16973.

src/common/docview.cpp

@@ -1147,17 +1147,21 @@ void wxDocManager::OnFileSaveAs(wxCommandEvent& WXUNUSED(event))
 
 void wxDocManager::OnMRUFile(wxCommandEvent& event)
 {
+    if ( m_fileHistory )
+    {
         // Check if the id is in the range assigned to MRU list entries.
         const int id = event.GetId();
         if ( id >= wxID_FILE1 &&
                 id < wxID_FILE1 + static_cast<int>(m_fileHistory->GetCount()) )
         {
             DoOpenMRUFile(id - wxID_FILE1);
+
+            // Don't skip the event below.
+            return;
         }
-    else
-    {
+    }
+
     event.Skip();
-    }
 }
 
 void wxDocManager::DoOpenMRUFile(unsigned n)