changed PLAINUI_BASE_URL to a default that doesn't crash when missing

src/rc3platform/settings/base.py

@@ -320,7 +320,7 @@ WORKADVENTURE_BACKEND_PUSH_ON_MAPSERVICE_DATA = True
 # ----------------------------------
 
 # base domain of the frontend (used to generate links in API and BackOffice, not in the plain UI itself)
-PLAINUI_BASE_URL = os.getenv('PLAINUI_BASE_URL', default=None)
+PLAINUI_BASE_URL = os.getenv('PLAINUI_BASE_URL', default='')
 
 # Database ID of the active conference for the PlainUI.
 # To initialize create a conference in the backend, then configure the conference, then restart the UI workers