fix: Always set _spotlight_url (#4186)

The conditional early exit in `SpotlightMiddleware` may cause attribute access errors when trying to check if `_spotlight_url` is set or not. This patch sets it to `None` explicitly at class level.

Author: BYK

sentry_sdk/spotlight.py

@@ -82,6 +82,7 @@ def capture_envelope(self, envelope):
 
     class SpotlightMiddleware(MiddlewareMixin):  # type: ignore[misc]
         _spotlight_script = None  # type: Optional[str]
+        _spotlight_url = None  # type: Optional[str]
 
         def __init__(self, get_response):
             # type: (Self, Callable[..., HttpResponse]) -> None
@@ -103,7 +104,7 @@ def __init__(self, get_response):
         @property
         def spotlight_script(self):
             # type: (Self) -> Optional[str]
-            if self._spotlight_script is None:
+            if self._spotlight_url is not None and self._spotlight_script is None:
                 try:
                     spotlight_js_url = urllib.parse.urljoin(
                         self._spotlight_url, SPOTLIGHT_JS_ENTRY_PATH
@@ -173,7 +174,7 @@ def process_response(self, _request, response):
 
         def process_exception(self, _request, exception):
             # type: (Self, HttpRequest, Exception) -> Optional[HttpResponseServerError]
-            if not settings.DEBUG:
+            if not settings.DEBUG or not self._spotlight_url:
                 return None
 
             try: