Fix two type errors

src/patito/exceptions.py

@@ -12,7 +12,7 @@
 class ValidationError(ValueError):
     """Exception raised when dataframe does not match schema."""
 
-    def __init__(self, errors: Sequence["ErrorWrapper"], model: Model) -> None:
+    def __init__(self, errors: Sequence["ErrorWrapper"], model: type[Model]) -> None:
         self.raw_errors = errors
         self.model = model
 

src/patito/pydantic.py

@@ -1151,6 +1151,11 @@ def join(
         ):
             for field_name, field in model.model_fields.items():
                 field_annotation = field.annotation
+                # Fix type error that field_annotation has type `type | None`,
+                # we want field_annotation as either `type` or `type | Type[None]`,
+                # where the latter means nullable
+                assert field_annotation is not None, "Column type cannot be null!"
+
                 if how in nullable_methods and type(None) not in get_args(
                     field_annotation
                 ):