Clear plugin webapp routes in default stop()

maubot/__init__.py

@@ -1,2 +1,3 @@
 from .plugin_base import Plugin
+from .plugin_server import PluginWebApp
 from .matrix import MaubotMatrixClient as Client, MaubotMessageEvent as MessageEvent

maubot/plugin_base.py

@@ -62,6 +62,8 @@ class Plugin(ABC):
     async def stop(self) -> None:
         for func, event_type in self._handlers_at_startup:
             self.client.remove_event_handler(event_type, func)
+        if self.webapp:
+            self.webapp.clear()
 
     @classmethod
     def get_config_class(cls) -> Optional[Type['BaseProxyConfig']]: