Fix: prevent crash when plist parsing returns non-dict value in userDefaults

[nilamwinarti13]

Some userDefaults.plist files cannot be parsed by plistlib or biplist and
return a string or None instead of a dictionary. The original code attempted
to iterate with plist.items() unconditionally, which caused:

AttributeError: 'str' object has no attribute 'items'

This patch adds a minimal type guard:

if not isinstance(plist, dict):
    continue

This ensures only valid plist dictionaries are processed, prevents
runtime crashes, and keeps existing functionality unchanged.