Fix getNextStreams from Youtube's channel extractor

src/main/java/org/schabi/newpipe/extractor/Extractor.java

@@ -66,6 +66,10 @@ public abstract class Extractor {
         if(!pageFetched) throw new IllegalStateException("Page is not fetched. Make sure you call fetchPage()");
     }
 
+    protected boolean isPageFetched() {
+        return pageFetched;
+    }
+
     /**
      * Fetch the current page.
      * @param downloader the download to use

src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeChannelExtractor.java

@@ -52,10 +52,18 @@ public class YoutubeChannelExtractor extends ChannelExtractor {
      */
     private Document nextStreamsAjax;
 
+    /**
+     * Unfortunately, we have to fetch the page even if we are only getting next streams,
+     * as they don't deliver enough information on their own (the channel name, for example).
+     * <br/>
+     * This help us to keep track on what are we fetching.
+     */
     private boolean fetchingNextStreams;
 
     public YoutubeChannelExtractor(StreamingService service, String url, String nextStreamsUrl) throws IOException, ExtractionException {
         super(service, url, nextStreamsUrl);
+
+        fetchingNextStreams = nextStreamsUrl != null && !nextStreamsUrl.isEmpty();
     }
 
     @Override
@@ -168,10 +176,11 @@ public class YoutubeChannelExtractor extends ChannelExtractor {
             throw new ExtractionException("Channel doesn't have more streams");
         }
 
-        StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());
+        if (!isPageFetched()) {
+            fetchPage();
+        }
 
-        //TODO: This is a horrible hack and should be fixed (the whole F*** architecture of ListExtractor is broken)
-        onFetchPage(NewPipe.getDownloader());
+        StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());
 
         setupNextStreamsAjax(NewPipe.getDownloader());
         collectStreamsFrom(collector, nextStreamsAjax.select("body").first());