Edit book: Fix typo that caused error popup while creating a new tag

src/calibre/gui2/tweak_book/editor/smart/html.py

@@ -90,7 +90,7 @@ def find_closing_tag(tag, max_tags=sys.maxint):
         if block is None or not tag_start.is_start:
             break
         endblock, tag_end = next_tag_boundary(block, tag_start.offset)
-        if block is None or tag_end.is_start:
+        if endblock is None or tag_end.is_start:
             break
         if tag_start.closing:
             try: