Actually there is no need to use flatMap

`null` values returned in the lambda are converted to empty `Optional`s in the `map` method: https://developer.android.com/reference/java/util/Optional#map(java.util.function.Function%3C?%20super%20T,%20?%20extends%20U%3E)

Author: Stypox

app/src/main/java/org/schabi/newpipe/RouterActivity.java

@@ -87,6 +87,7 @@
 import org.schabi.newpipe.views.FocusOverlayView;
 
 import java.io.Serializable;
+import java.lang.ref.Reference;
 import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -748,7 +749,7 @@ public void onDestroy() {
          */
         private Optional<AppCompatActivity> getActivityContext() {
             return Optional.ofNullable(weakContext)
-                    .flatMap(context -> Optional.ofNullable(context.get()))
+                    .map(Reference::get)
                     .filter(context -> !context.isFinishing());
         }
 

app/src/main/java/org/schabi/newpipe/fragments/detail/VideoDetailFragment.java

@@ -1887,7 +1887,7 @@ public void onFullscreenStateChanged(final boolean fullscreen) {
         setupBrightness();
         if (!isPlayerAndPlayerServiceAvailable()
                 || player.UIs().get(MainPlayerUi.class).isEmpty()
-                || getRoot().flatMap(v -> Optional.ofNullable(v.getParent())).isEmpty()) {
+                || getRoot().map(View::getParent).isEmpty()) {
             return;
         }
 

app/src/main/java/org/schabi/newpipe/player/mediaitem/MediaItemTag.java

@@ -62,8 +62,8 @@ default Optional<Quality> getMaybeQuality() {
     @NonNull
     static Optional<MediaItemTag> from(@Nullable final MediaItem mediaItem) {
         return Optional.ofNullable(mediaItem)
-                .flatMap(item -> Optional.ofNullable(item.localConfiguration))
-                .flatMap(localConfiguration -> Optional.ofNullable(localConfiguration.tag))
+                .map(item -> item.localConfiguration)
+                .map(localConfiguration -> localConfiguration.tag)
                 .filter(MediaItemTag.class::isInstance)
                 .map(MediaItemTag.class::cast);
     }