raising error if plugin not initialized (#15319)

api/core/plugin/entities/plugin.py

 from typing import Any, Optional
 
 from pydantic import BaseModel, Field, model_validator
+from werkzeug.exceptions import NotFound
 
 from core.agent.plugin_entities import AgentStrategyProviderEntity
 from core.model_runtime.entities.provider_entities import ProviderEntity
         return f"{self.organization}/{self.plugin_name}/{self.provider_name}"
 
     def __init__(self, value: str, is_hardcoded: bool = False) -> None:
+        if not value:
+            raise NotFound("plugin not found, please add plugin")
         # check if the value is a valid plugin id with format: $organization/$plugin_name/$provider_name
         if not re.match(r"^[a-z0-9_-]+\/[a-z0-9_-]+\/[a-z0-9_-]+$", value):
             # check if matches [a-z0-9_-]+, if yes, append with langgenius/$value/$value