Stubgen crashes on new Python 3.10 union syntax

[KapJI]

Crash Report

It works in some cases e.g. with cast() function but here with TypeVar it crashes.

Mypy accepts this file with no errors.

Traceback

  File "mypy/stubgen.py", line 1858, in main
  File "mypy/stubgen.py", line 1701, in generate_stubs
  File "mypy/stubgen.py", line 1643, in generate_stub_from_ast
  File "mypy/nodes.py", line 373, in accept
  File "mypy/stubgen.py", line 643, in visit_mypy_file
  File "mypy/traverser.py", line 114, in visit_mypy_file
  File "mypy/nodes.py", line 1301, in accept
  File "mypy/stubgen.py", line 1006, in visit_assignment_stmt
  File "mypy/stubgen.py", line 1117, in process_typealias
  File "mypy/nodes.py", line 1841, in accept
  File "mypy/stubgen.py", line 362, in visit_call_expr
TypeError: str object expected; got None

To Reproduce

from __future__ import annotations

from typing import TypeVar

T = TypeVar("T", bound=str | None)

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: stubgen test.py
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.10.9
• Operating system and version: macOS