text: Title: Addon error: Flow is not killable
  Body: #### Problem Description
  
  ~~~
  > mitmproxy -r sky-phone-login.txt
  Errors logged during startup:
  Addon error: Flow is not killable.
  Traceback (most recent call last):
    File "mitmproxy\addons\disable_h2c.py", line 41, in request
    File "mitmproxy\addons\disable_h2c.py", line 33, in process_flow
    File "mitmproxy\flow.py", line 235, in kill
  mitmproxy.exceptions.ControlException: Flow is not killable.
  ~~~
  
  #### Steps to reproduce the behavior:
  
  ~~~
  : save.file @shown sky-phone-login.txt
  ~~~
  
  #### System Information
  
  ~~~
  Mitmproxy: 10.3.0 binary
  Python:    3.12.3
  OpenSSL:   OpenSSL 3.2.1 30 Jan 2024
  Platform:  Windows-10-10.0.18363-SP0
  ~~~