fix DownloadDialog crash if there are no streams available yet

If there are no streams the DownloadDialog will close again quickly,
but the rxJava job fetching the VideoSegments is still going.
This patch fixes that:
- move initialization of getting VideoSegments rxJava job until after initToolbar()
- have Disposable for the VideoSegments stuff that will be disposed if the
  DownloadDialog will be destroyed.

app/src/main/java/org/schabi/newpipe/download/DownloadDialog.java

@@ -87,6 +87,7 @@ import icepick.State;
 import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers;
 import io.reactivex.rxjava3.core.Single;
 import io.reactivex.rxjava3.disposables.CompositeDisposable;
+import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.schedulers.Schedulers;
 import us.shandian.giga.get.MissionRecoveryInfo;
 import us.shandian.giga.postprocessing.Postprocessing;
@@ -153,6 +154,8 @@ public class DownloadDialog extends DialogFragment
     private final ActivityResultLauncher<Intent> requestDownloadPickVideoFolderLauncher =
             registerForActivityResult(
                     new StartActivityForResult(), this::requestDownloadPickVideoFolderResult);
+    @NonNull
+    private Disposable youtubeVideoSegmentsDisposable;
 
 
     /*//////////////////////////////////////////////////////////////////////////
@@ -254,8 +257,6 @@ public class DownloadDialog extends DialogFragment
                 downloadManager = mgr.getDownloadManager();
                 askForSavePath = mgr.askForSavePath();
 
-                checkForYoutubeVideoSegments();
-
                 context.unbindService(this);
             }
 
@@ -332,6 +333,7 @@ public class DownloadDialog extends DialogFragment
         showLoading();
 
         initToolbar(dialogBinding.toolbarLayout.toolbar);
+        checkForYoutubeVideoSegments();
         setupDownloadOptions();
 
         prefs = PreferenceManager.getDefaultSharedPreferences(requireContext());
@@ -393,6 +395,7 @@ public class DownloadDialog extends DialogFragment
 
     @Override
     public void onDestroyView() {
+        youtubeVideoSegmentsDisposable.dispose();
         dialogBinding = null;
         super.onDestroyView();
     }
@@ -1158,7 +1161,7 @@ public class DownloadDialog extends DialogFragment
     }
 
     private void checkForYoutubeVideoSegments() {
-        disposables.add(Single.fromCallable(() -> {
+        youtubeVideoSegmentsDisposable = Single.fromCallable(() -> {
                     VideoSegment[] videoSegments = null;
                     try {
                         videoSegments = SponsorBlockUtils
@@ -1177,7 +1180,7 @@ public class DownloadDialog extends DialogFragment
                     setVideoSegments(videoSegments);
                     okButton.setEnabled(true);
                     hideLoading();
-                }));
+                });
     }
 
     public void showLoading() {