fix: error when overwriting a sub table

Fix #313

Signed-off-by: Frost Ming <[email protected]>

Author: frostming

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed a bug that overwriting a sub table with a plain value raises an error. ([#313](https://github.com/sdispater/tomlkit/issues/313))
+
 ## [0.12.2] - 2023-09-20
 
 ### Fixed

tomlkit/items.py

@@ -1945,7 +1945,7 @@ class Null(Item):
     """
 
     def __init__(self) -> None:
-        pass
+        super().__init__(Trivia(trail=""))
 
     def unwrap(self) -> None:
         return None