fix: guard against None model in call_utility_model to prevent crash

[gdeyoung]

TL;DR — Agent crashes when utility model resolution returns None

call_utility_model() gets the model name from get_utility_model() but never checks if it's None. If model resolution fails (missing config, bad provider name, etc.), the agent crashes with an unhandled exception. A simple None guard logs a warning and returns an empty string, allowing the agent to continue operating.

One defensive check prevents a crash when model configuration is incomplete.

---

Problem

In agent.py, call_utility_model() at line ~759:

model = self.get_utility_model()

# Immediately used without checking if None
# call extensions
call_data = {"model": model, ...}

When get_utility_model() returns None (e.g., missing utility_model in settings, provider not configured), the agent crashes instead of gracefully degrading.

Solution

Add a None guard immediately after model resolution:

model = self.get_utility_model()

if model is None:
    self.context.log.log(
        type="warning",
        heading="Utility model unavailable",
        content="build_utility_model returned None — skipping utility call.",
    )
    return ""

The agent logs a warning and continues operating instead of crashing.

Impact

• Before: Agent crashes when utility model is misconfigured or unavailable
• After: Agent logs warning, skips the utility call, continues operating
• Common trigger: Fresh installs, config migration, provider outages

Changes

File	Change
agent.py	Add if model is None guard after get_utility_model()

Testing

• 2 regression tests verify None model returns empty string
• 21/21 tests passing across all critical patches

Related

• PR fix: prevent infinite cascade crash by converting fw.warning/fw.error to plain text #1457 (cascade fix)
• PR fix: graceful recovery from malformed LLM tool call output #1458 (validation recovery)
• PR feat: add regex fallback parser to recover malformed JSON tool calls #1459 (regex fallback)
• Issue # Agent Zero JSON Malformation Bug Fixes #1031 (JSON Malformation Bug Fixes)