Fix issue #997 - Store endpoint and create it only if it changes

app/src/main/java/app/fedilab/android/mastodon/services/CustomReceiver.java

@@ -66,14 +66,16 @@ public class CustomReceiver extends MessagingReceiver {
     @Override
     public void onNewEndpoint(@Nullable Context context, @NotNull String endpoint, @NotNull String slug) {
         if (context != null) {
-            SharedPreferences sharedpreferences = PreferenceManager.getDefaultSharedPreferences(context);
-            String storedEnpoint = sharedpreferences.getString(context.getString(R.string.SET_STORED_ENDPOINT)+slug, null);
-            if(storedEnpoint == null || !storedEnpoint.equalsIgnoreCase(endpoint)) {
-                PushNotifications
-                        .registerPushNotifications(context, endpoint, slug);
-                SharedPreferences.Editor editor = sharedpreferences.edit();
-                editor.putString(context.getString(R.string.SET_STORED_ENDPOINT)+slug, endpoint);
-                editor.apply();
+            synchronized(this) {
+                SharedPreferences sharedpreferences = PreferenceManager.getDefaultSharedPreferences(context);
+                String storedEnpoint = sharedpreferences.getString(context.getString(R.string.SET_STORED_ENDPOINT)+slug, null);
+                if(storedEnpoint == null || !storedEnpoint.equals(endpoint)) {
+                    PushNotifications
+                            .registerPushNotifications(context, endpoint, slug);
+                    SharedPreferences.Editor editor = sharedpreferences.edit();
+                    editor.putString(context.getString(R.string.SET_STORED_ENDPOINT)+slug, endpoint);
+                    editor.commit();
+                }
             }
         }
     }