Fix error when changing the style of misspelt words

In the note editor, changing the style of text failing the spell
check caused an error.  The style tag used by the spell checker
to indicate spelling errors has no tag name.

Fixes #13282.

gramps/gui/widgets/styledtextbuffer.py

@@ -492,7 +492,7 @@ def _remove_style_from_selection(self, style):
         start, end = self._get_selection()
         tags = self._get_tag_from_range(start.get_offset(), end.get_offset())
         for tag_name, tag_data in tags.items():
-            if tag_name.startswith(str(style)):
+            if tag_name is not None and tag_name.startswith(str(style)):
                 for start, end in tag_data:
                     self.remove_tag_by_name(
                         tag_name,