Avoid saving a non populated header state for the table view headers

This was causing problems loading content on the table view that could persist between sessions.

YACReaderLibrary/classic_comics_view.cpp

@@ -353,6 +353,10 @@ void ClassicComicsView::updateTableView(int i)
 
 void ClassicComicsView::saveTableHeadersStatus()
 {
+    if (model == nullptr) {
+        return;
+    }
+
     settings->setValue(COMICS_VIEW_HEADERS, tableView->horizontalHeader()->saveState());
 }