Fix NPE in BackupFragment when fragment exits while refresh is happening

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

src/main/java/com/owncloud/android/ui/fragment/contactsbackup/BackupFragment.java

@@ -267,11 +267,11 @@ public class BackupFragment extends FileFragment implements DatePickerDialog.OnD
 
             @Override
             protected void onPostExecute(Boolean result) {
-                if (result) {
+                if (result && binding != null) {
                     OCFile backupFolder = storageManager.getFileByPath(backupFolderPath);
 
                     List<OCFile> backupFiles = storageManager
-                            .getFolderContent(backupFolder, false);
+                        .getFolderContent(backupFolder, false);
 
                     Collections.sort(backupFiles, new AlphanumComparator<>());