Better error message when ToC editor is opened to an non-HTML file

src/calibre/gui2/toc/location.py

@@ -397,6 +397,8 @@ def current_changed(self, item):
         name = self.current_name = str(item.data(Qt.ItemDataRole.DisplayRole) or '')
         # Ensure encoding map is populated
         root = self.container.parsed(name)
+        if not hasattr(root, 'xpath'):
+            return error_dialog(self, _('Not an HTML file'), _('The file {} is not marked as an HTML file in the OPF and cannot be displayed').format(name))
         nasty = root.xpath('//*[local-name()="head"]/*[local-name()="p"]')
         if nasty:
             body = root.xpath('//*[local-name()="body"]')