TypeError in LoadSkillTool.run_async after session rewind: 'NoneType' object is not iterable

[SAMFVH]

Bug Report

Description

LoadSkillTool.run_async crashes with TypeError: 'NoneType' object is not iterable after a session rewind when the skills feature is enabled.

Steps to Reproduce

1. Enable the SKILL_TOOLSET experimental feature
2. Run an agent session that activates a skill (populating _adk_activated_skill_{agent_name} in state)
3. Rewind the session to a point before the skill was activated
4. Continue the session and trigger a load_skill tool call

Root Cause

Runner._compute_state_delta_for_rewind (runners.py:716) sets state keys to None to mark them for deletion when they were added after the rewind point:

# runners.py:715-716
if key not in state_at_rewind_point:
    rewind_state_delta[key] = None

This sets _adk_activated_skill_{agent_name} to None. Later, LoadSkillTool.run_async does:

# skill_toolset.py:155
activated_skills = list(tool_context.state.get(state_key, []))

Since the key exists with value None, dict.get() returns None (not the default []), and list(None) raises TypeError.

Stack Trace

File ".../google/adk/tools/skill_toolset.py", line 155, in run_async
    activated_skills = list(tool_context.state.get(state_key, []))
TypeError: 'NoneType' object is not iterable

Suggested Fix

# skill_toolset.py:155
activated_skills = list(tool_context.state.get(state_key) or [])

This handles both missing keys and explicit None values. The same pattern should be applied to _resolve_additional_tools_from_state (line 742) for consistency, even though it currently doesn't crash due to the if not activated_skills guard on line 744.

Version

google-adk 1.3.0

[enjoykumawat]

I'd like to work on this. Adding a None guard before iterating in LoadSkillTool.run_async. Will submit a PR shortly.

[surajksharma07]

@SAMFVH The rewind path sets adk_activated_skill{agent_name} to None as a deletion marker, and dict.get(key, default) only falls back to the default when the key is absent — not when it's explicitly None — so list(None) blows up.

As a quick workaround, try swapping line 155 in skill_toolset.py from state.get(state_key, []) to state.get(state_key) or [] and see if that clears the crash on your end.

Commit cb5d1f8 is already heading in this direction, but it'd be great if you could test it thoroughly on your setup.

[SAMFVH]

We ran into this in production. Our workaround patches all 3 crash sites from our side:

1. LoadSkillTool.run_async (line 155) — subclassed LoadSkillTool to normalize None → [] in state before calling super().run_async()
2. SkillToolset._resolve_additional_tools_from_state (line 742) — overrode in our SkillToolset subclass to normalize via the underlying session.state (since ReadonlyContext.state is a MappingProxyType)
3. Our own activate_skill_for_step that also reads the same state key — switched from list(state.get(key, [])) to list(raw) if raw is not None else []

Happy to see this fixed upstream so we can drop the workarounds. The suggested state.get(key) or [] would work for all 3 sites.