[orca] Add null check in _generateColumnHeaderIfToggleAndNoText

[Joanmarie Diggs <joanied src gnome org>]

commit f58697766554fe9639f6c7d699aafa75998477a7
Author: Joanmarie Diggs <jdiggs igalia com>
Date:   Mon Oct 17 11:47:30 2022 +0200

    Add null check in _generateColumnHeaderIfToggleAndNoText
    
    columnHeaderForCell can now return None in the case of error conditions.
    Thus we need to check for that in the generator method.
    
    See issue #274

 src/orca/generator.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/orca/generator.py b/src/orca/generator.py
index b1a768117..9c8a127f2 100644
--- a/src/orca/generator.py
+++ b/src/orca/generator.py
@@ -957,7 +957,8 @@ class Generator:
         label = self._script.utilities.displayedText(descendant)
         if not label and self._script.utilities.hasMeaningfulToggleAction(obj):
             accHeader = self._script.utilities.columnHeaderForCell(obj)
-            result.append(accHeader.name)
+            if (accHeader):
+                result.append(accHeader.name)
         return result
 
     def _generateRealTableCell(self, obj, **args):