ensure emulator is used before using remote config

Author: kroikie

firebase-config/src/main/java/com/google/firebase/remoteconfig/FirebaseRemoteConfig.java

@@ -23,6 +23,8 @@
 import com.google.android.gms.tasks.Task;
 import com.google.android.gms.tasks.Tasks;
 import com.google.firebase.FirebaseApp;
+import com.google.firebase.FirebaseError;
+import com.google.firebase.FirebaseException;
 import com.google.firebase.abt.AbtException;
 import com.google.firebase.abt.FirebaseABTesting;
 import com.google.firebase.emulators.EmulatedServiceSettings;
@@ -580,8 +582,14 @@ public ConfigUpdateListenerRegistration addOnConfigUpdateListener(
    *
    * @param host the emulator host (for example, 10.0.2.2)
    * @param port the emulator port (for example, 9299)
+   *
+   * @exception FirebaseRemoteConfigClientException Thrown when useEmulator is called after a fetch.
    */
-  public void useEmulator(@NonNull String host, int port) {
+  public void useEmulator(@NonNull String host, int port) throws FirebaseRemoteConfigClientException {
+    if (this.frcMetadata.getInfo().getLastFetchStatus() != LAST_FETCH_STATUS_NO_FETCH_YET) {
+      throw new FirebaseRemoteConfigClientException(
+              "Cannot connect to emulator after a fetch has been made.");
+    }
     fetchHandler.setEmulatedServiceSettings(new EmulatedServiceSettings(host, port));
   }