fix(hashtag-fragment): fix crash when opening some hashtags

cc: @sk22

mastodon/src/main/java/org/joinmastodon/android/fragments/HashtagTimelineFragment.java

@@ -85,7 +85,7 @@ public class HashtagTimelineFragment extends PinnableStatusListFragment{
 
 	@Override
 	protected TimelineDefinition makeTimelineDefinition() {
-		return TimelineDefinition.ofHashtag(hashtag);
+		return TimelineDefinition.ofHashtag(hashtagName);
 	}
 
 	@Override