Fix App Check crash if there is a config problem (#878)

Author: a-maurice

app_check/src/FirebaseAppCheck.cs

@@ -162,12 +162,14 @@ private static void GetTokenFromCSharpMethod(string appName, int key) {
       AppCheckUtil.FinishGetTokenCallback(key, "", 0,
         (int)AppCheckError.InvalidConfiguration,
         "Missing IAppCheckProviderFactory.");
+      return;
     }
     FirebaseApp app = FirebaseApp.GetInstance(appName);
     if (app == null) {
       AppCheckUtil.FinishGetTokenCallback(key, "", 0,
         (int)AppCheckError.Unknown,
         "Unable to find App with name: " + appName);
+      return;
     }
     IAppCheckProvider provider;
     if (!providerMap.TryGetValue(app.Name, out provider)) {
@@ -176,6 +178,7 @@ private static void GetTokenFromCSharpMethod(string appName, int key) {
         AppCheckUtil.FinishGetTokenCallback(key, "", 0,
           (int)AppCheckError.InvalidConfiguration,
           "Failed to create IAppCheckProvider for App: " + appName);
+        return;
       }
       providerMap[app.Name] = provider;
     }

docs/readme.md

@@ -71,6 +71,11 @@ Support
 
 Release Notes
 -------------
+### Upcoming
+- Changes
+    - App Check: Fixed a crash when there are errors creating a provider.
+      ([#877](https://github.com/firebase/firebase-unity-sdk/issues/877))
+
 ### 11.5.0
 - Changes
     - General: Update to Firebase C++ SDK version 11.5.0.