Fix possible exception.

diff --git a/source/NVDAObjects/IAccessible/mozilla.py b/source/NVDAObjects/IAccessible/mozilla.py
index 14a4287..974efa7 100755 (executable)
--- a/source/NVDAObjects/IAccessible/mozilla.py
+++ b/source/NVDAObjects/IAccessible/mozilla.py
@@ -227,7 +227,7 @@ def findExtraOverlayClasses(obj, clsList):
                # Check if the tree view is a table.\r
                parent = obj.parent\r
                # Tree view items may be nested, so skip any tree view item ancestors.\r
-               while parent and parent.IAccessibleRole == oleacc.ROLE_SYSTEM_OUTLINEITEM:\r
+               while parent and isinstance(parent, Mozilla) and parent.IAccessibleRole == oleacc.ROLE_SYSTEM_OUTLINEITEM:\r
                        newParent = parent.parent\r
                        parent.parent = newParent\r
                        parent = newParent\r