Check for AppCheck's existance before subscribing (#8898)

Author: schmidt-sebastian

Firestore/CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+- [fixed] Fixed a bug in the AppCheck integration that caused the SDK to respond
+  to unrelated notifications (#8895).
+
 # v8.9.0
 - [added] Added support for Firebase AppCheck.
 - [fixed] Fix a crash if `[FIRTransaction getDocument]` was called after

Firestore/core/src/credentials/firebase_app_check_credentials_provider_apple.mm

@@ -31,8 +31,12 @@
 FirebaseAppCheckCredentialsProvider::FirebaseAppCheckCredentialsProvider(
     FIRApp* app, id<FIRAppCheckInterop> app_check) {
   contents_ = std::make_shared<Contents>(app, app_check);
-  std::weak_ptr<Contents> weak_contents = contents_;
 
+  if (app_check == nil) {
+    return;
+  }
+
+  std::weak_ptr<Contents> weak_contents = contents_;
   app_check_listener_handle_ = [[NSNotificationCenter defaultCenter]
       addObserverForName:[app_check tokenDidChangeNotificationName]
                   object:nil