Fix task configuration with None context parameter in YAML

Fixes #3696

When context: None is specified in YAML, yaml.safe_load() converts it to
the string 'None' instead of Python's None object. The code was attempting
to iterate over this string character by character, causing KeyError: 'N'
when trying to look up task names.

Changes:
- Added isinstance(context_list, list) check in crew_base.py before
  iterating context_list to handle YAML's conversion of None to string
- Added test case test_task_with_none_context_from_yaml to verify tasks
  can be configured with context: None without errors
- Added test YAML configurations in tests/config_none_context/ to
  reproduce and verify the fix

The fix ensures that only actual list values are processed, allowing
None and other non-list values to pass through without causing errors.

Co-Authored-By: João <joao@crewai.com>

src/crewai/project/crew_base.py

@@ -260,9 +260,10 @@ def CrewBase(cls: T) -> T:  # noqa: N802
             output_pydantic_functions: dict[str, Callable],
         ) -> None:
             if context_list := task_info.get("context"):
-                self.tasks_config[task_name]["context"] = [
-                    tasks[context_task_name]() for context_task_name in context_list
-                ]
+                if isinstance(context_list, list):
+                    self.tasks_config[task_name]["context"] = [
+                        tasks[context_task_name]() for context_task_name in context_list
+                    ]
 
             if tools := task_info.get("tools"):
                 self.tasks_config[task_name]["tools"] = [

tests/config_none_context/agents.yaml

@@ -0,0 +1,5 @@
+test_agent:
+  role: Test Agent
+  goal: Test goal
+  backstory: Test backstory
+  verbose: true

tests/config_none_context/tasks.yaml

@@ -0,0 +1,11 @@
+task_with_none_context:
+  description: A test task with None context
+  expected_output: Some output
+  agent: test_agent
+  context: None
+
+task_with_valid_context:
+  description: A test task with valid context
+  expected_output: Some output
+  agent: test_agent
+  context: [task_with_none_context]

tests/test_project.py

@@ -287,3 +287,38 @@ def test_internal_crew_with_mcp():
     adapter_mock.assert_called_once_with(
         {"host": "localhost", "port": 8000}, connect_timeout=120
     )
+
+
+def test_task_with_none_context_from_yaml():
+    @CrewBase
+    class CrewWithNoneContext:
+        agents_config = "config_none_context/agents.yaml"
+        tasks_config = "config_none_context/tasks.yaml"
+
+        agents: list[BaseAgent]
+        tasks: list[Task]
+
+        @agent
+        def test_agent(self):
+            return Agent(config=self.agents_config["test_agent"])
+
+        @task
+        def task_with_none_context(self):
+            return Task(config=self.tasks_config["task_with_none_context"])
+
+        @task
+        def task_with_valid_context(self):
+            return Task(config=self.tasks_config["task_with_valid_context"])
+
+        @crew
+        def crew(self):
+            return Crew(agents=self.agents, tasks=self.tasks, verbose=True)
+
+    crew_instance = CrewWithNoneContext()
+
+    task_none = crew_instance.task_with_none_context()
+    assert task_none is not None
+
+    task_valid = crew_instance.task_with_valid_context()
+    assert task_valid.context is not None
+    assert len(task_valid.context) == 1