prevent null-pointer in case remote file listing UI gets closed before the async loading comes back

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

app/src/main/java/com/nextcloud/talk/components/filebrowser/operations/DavListing.java

@@ -2,6 +2,8 @@
  * Nextcloud Talk application
  *
  * @author Mario Danic
+ * @author Andy Scherzinger
+ * Copyright (C) 2022 Andy Scherzinger <info@andy-scherzinger.de>
  * Copyright (C) 2017-2018 Mario Danic <mario@lovelyhq.com>
  *
  * This program is free software: you can redistribute it and/or modify
@@ -20,20 +22,26 @@
 
 package com.nextcloud.talk.components.filebrowser.operations;
 
-import androidx.annotation.Nullable;
+import android.util.Log;
+
 import com.nextcloud.talk.components.filebrowser.interfaces.ListingInterface;
 import com.nextcloud.talk.components.filebrowser.models.DavResponse;
 import com.nextcloud.talk.components.filebrowser.webdav.ReadFilesystemOperation;
 import com.nextcloud.talk.models.database.UserEntity;
+
+import java.util.concurrent.Callable;
+
+import androidx.annotation.Nullable;
 import io.reactivex.Single;
 import io.reactivex.SingleObserver;
+import io.reactivex.annotations.NonNull;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.schedulers.Schedulers;
 import okhttp3.OkHttpClient;
 
-import java.util.concurrent.Callable;
-
 public class DavListing extends ListingAbstractClass {
+    private static final String TAG = DavListing.class.getSimpleName();
+
     private DavResponse davResponse = new DavResponse();
 
     public DavListing(ListingInterface listingInterface) {
@@ -50,18 +58,22 @@ public class DavListing extends ListingAbstractClass {
         }).subscribeOn(Schedulers.io())
                 .subscribe(new SingleObserver<ReadFilesystemOperation>() {
                     @Override
-                    public void onSubscribe(Disposable d) {
+                    public void onSubscribe(@NonNull Disposable d) {
 
                     }
 
                     @Override
-                    public void onSuccess(ReadFilesystemOperation readFilesystemOperation) {
+                    public void onSuccess(@NonNull ReadFilesystemOperation readFilesystemOperation) {
                         davResponse = readFilesystemOperation.readRemotePath();
-                        listingInterface.listingResult(davResponse);
+                        try {
+                            listingInterface.listingResult(davResponse);
+                        } catch (NullPointerException npe) {
+                            Log.i(TAG, "Error loading remote folder - due to view already been terminated", npe);
+                        }
                     }
 
                     @Override
-                    public void onError(Throwable e) {
+                    public void onError(@NonNull Throwable e) {
                         listingInterface.listingResult(davResponse);
                     }
                 });