[YouTube] Fix extraction of continuations in initial playlist responses

Author: AudricV

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubePlaylistExtractor.java

@@ -374,22 +374,47 @@ private Page getNextPageFrom(final JsonArray contents)
 
         final JsonObject lastElement = contents.getObject(contents.size() - 1);
         if (lastElement.has("continuationItemRenderer")) {
-            final String continuation = lastElement
+            final JsonObject continuationEndpoint = lastElement
                     .getObject("continuationItemRenderer")
-                    .getObject("continuationEndpoint")
-                    .getObject("continuationCommand")
+                    .getObject("continuationEndpoint");
+
+            final JsonObject continuationObject;
+            if (continuationEndpoint.has("commandExecutorCommand")) {
+                // This structure is only used at the time this code is written in initial playlist
+                // responses. continuationItemRenderer objects return multiple commands: one
+                // containing the continuation we need and one a playlistVotingRefreshPopupCommand
+                continuationObject = continuationEndpoint.getObject("commandExecutorCommand")
+                        .getArray("commands")
+                        .stream()
+                        .filter(JsonObject.class::isInstance)
+                        .map(JsonObject.class::cast)
+                        .filter(command -> command.has("continuationCommand"))
+                        .findFirst()
+                        .orElse(new JsonObject());
+            } else {
+                // At the time this code is written, this "classic" continuation structure is only
+                // returned in browse responses of continuation requests
+                continuationObject = continuationEndpoint;
+            }
+
+            final String continuation = continuationObject.getObject("continuationCommand")
                     .getString("token");
 
+            if (isNullOrEmpty(continuation)) {
+                // Invalid continuation or no continuation found
+                return null;
+            }
+
             final byte[] body = JsonWriter.string(prepareDesktopJsonBuilder(
                             getExtractorLocalization(), getExtractorContentCountry())
                             .value("continuation", continuation)
                             .done())
                     .getBytes(StandardCharsets.UTF_8);
 
             return new Page(YOUTUBEI_V1_URL + "browse?" + DISABLE_PRETTY_PRINT_PARAMETER, body);
-        } else {
-            return null;
         }
+
+        return null;
     }
 
     private void collectStreamsFrom(@Nonnull final StreamInfoItemsCollector collector,