checkdocs.py: fix NoneType error

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>

diff --git a/tools/checkdocs.py b/tools/checkdocs.py
index 9f56a3f2112e268c1eaf2d444a711bdcdadb53cb..3fc55865ce2f3e63291182dc54000c206c54e2fe 100755 (executable)
--- a/tools/checkdocs.py
+++ b/tools/checkdocs.py
@@ -101,6 +101,7 @@ def _check_file_links(toc_ids, path, c):
 
         if classes is None:
             _pwarn(path, 'External link has no "ext" class: "{}"'.format(link))
+            classes = []
         else:
             classes = classes.group(1).split(' ')