Fix crash on startup without internet: Cbservable callable returning null

Use Maybe instead

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

@@ -8,6 +8,7 @@ import android.os.Build;
 import android.util.Log;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 import androidx.multidex.MultiDexApplication;
 import androidx.preference.PreferenceManager;
 
@@ -67,7 +68,7 @@ public class App extends MultiDexApplication {
     protected static final String TAG = App.class.toString();
     private static App app;
 
-    private Disposable disposable = null;
+    @Nullable private Disposable disposable = null;
 
     @NonNull
     public static App getApp() {

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

@@ -12,6 +12,7 @@ import android.net.Uri;
 import android.util.Log;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 import androidx.core.app.NotificationCompat;
 import androidx.core.app.NotificationManagerCompat;
 import androidx.core.content.ContextCompat;
@@ -36,10 +37,9 @@ import java.security.cert.CertificateException;
 import java.security.cert.CertificateFactory;
 import java.security.cert.X509Certificate;
 
-import io.reactivex.Observable;
+import io.reactivex.Maybe;
 import io.reactivex.android.schedulers.AndroidSchedulers;
 import io.reactivex.disposables.Disposable;
-import io.reactivex.disposables.Disposables;
 import io.reactivex.schedulers.Schedulers;
 
 public final class CheckForNewAppVersion {
@@ -174,18 +174,18 @@ public final class CheckForNewAppVersion {
         return getCertificateSHA1Fingerprint(app).equals(GITHUB_APK_SHA1);
     }
 
-    @NonNull
+    @Nullable
     public static Disposable checkNewVersion(@NonNull final App app) {
         final SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(app);
 
         // Check if user has enabled/disabled update checking
         // and if the current apk is a github one or not.
-        if (!prefs.getBoolean(app.getString(R.string.update_app_key), true)
-                || !isGithubApk(app)) {
-            return Disposables.empty();
+        if (!prefs.getBoolean(app.getString(R.string.update_app_key), true) || !isGithubApk(app)) {
+            return null;
         }
 
-        return Observable.fromCallable(() -> {
+        return Maybe
+                .fromCallable(() -> {
                     if (!isConnected(app)) {
                         return null;
                     }
@@ -198,15 +198,13 @@ public final class CheckForNewAppVersion {
                         if (DEBUG) {
                             Log.w(TAG, Log.getStackTraceString(e));
                         }
-            }
-
                         return null;
+                    }
                 })
                 .subscribeOn(Schedulers.io())
                 .observeOn(AndroidSchedulers.mainThread())
                 .subscribe(response -> {
                     // Parse the json from the response.
-                    if (response != null) {
                     try {
                         final JsonObject githubStableObject = JsonParser.object().from(response)
                                 .getObject("flavors").getObject("github").getObject("stable");
@@ -223,7 +221,6 @@ public final class CheckForNewAppVersion {
                             Log.w(TAG, Log.getStackTraceString(e));
                         }
                     }
-                    }
                 });
     }
 }