Fix NullPointerException when sharing a playlist which is loading

Prevent a NullPointerException by adding a null check for currentInfo when sharing a playlist.

app/src/main/java/org/schabi/newpipe/fragments/list/playlist/PlaylistFragment.java

@@ -268,7 +268,10 @@ public class PlaylistFragment extends BaseListInfoFragment<PlaylistInfo> {
                 ShareUtils.openUrlInBrowser(requireContext(), url);
                 break;
             case R.id.menu_item_share:
-                ShareUtils.shareText(requireContext(), name, url, currentInfo.getThumbnailUrl());
+                if (currentInfo != null) {
+                    ShareUtils.shareText(requireContext(), name, url,
+                            currentInfo.getThumbnailUrl());
+                }
                 break;
             case R.id.menu_item_bookmark:
                 onBookmarkClicked();