Add default model for CrewEvaluator and fix event import order

- Set default model to "gpt-4o-mini" in CrewEvaluator when no model is specified
- Reorder event-related imports in task.py to follow standard import conventions
- Update event bus initialization method return type hint
- Export event_bus in events/__init__.py

src/crewai/crew.py

@@ -1209,7 +1209,7 @@ class Crew(BaseModel):
                 ),
             )
             test_crew = self.copy()
-            evaluator = CrewEvaluator(test_crew, openai_model_name)
+            evaluator = CrewEvaluator(test_crew, openai_model_name or "gpt-4o-mini")
 
             for i in range(1, n_iterations + 1):
                 evaluator.set_iteration(i)

src/crewai/task.py

@@ -40,12 +40,12 @@ from crewai.telemetry.telemetry import Telemetry
 from crewai.tools.base_tool import BaseTool
 from crewai.utilities.config import process_config
 from crewai.utilities.converter import Converter, convert_to_model
-from crewai.utilities.events.event_bus import event_bus
 from crewai.utilities.events import (
     TaskCompletedEvent,
     TaskFailedEvent,
     TaskStartedEvent,
 )
+from crewai.utilities.events.event_bus import event_bus
 from crewai.utilities.i18n import I18N
 from crewai.utilities.printer import Printer
 

src/crewai/utilities/events/__init__.py

@@ -15,7 +15,7 @@ from .flow_events import (
     MethodExecutionStartedEvent,
     MethodExecutionFinishedEvent,
 )
-from .event_bus import EventTypes, EventBus
+from .event_bus import EventTypes, EventBus, event_bus
 from .tool_usage_events import ToolUsageFinishedEvent, ToolUsageErrorEvent
 
 # events

src/crewai/utilities/events/event_bus.py

@@ -28,7 +28,7 @@ class EventBus:
                     cls._instance._initialize()
         return cls._instance
 
-    def _initialize(self):
+    def _initialize(self) -> None:
         """Initialize the event bus internal state"""
         self._signal = Signal("event_bus")
         self._handlers: Dict[