fix NPE in dateFormat (#2549)

Bug #2548 fix NPE in dateFormat pref

app/src/main/java/eu/kanade/tachiyomi/data/preference/PreferencesHelper.kt

@@ -145,7 +145,7 @@ class PreferencesHelper(val context: Context) {
 
     fun backupsDirectory() = rxPrefs.getString(Keys.backupDirectory, defaultBackupDir.toString())
 
-    fun dateFormat() = rxPrefs.getObject(Keys.dateFormat, DateFormatConverter())
+    fun dateFormat() = rxPrefs.getObject(Keys.dateFormat, DateFormat.getDateInstance(DateFormat.SHORT), DateFormatConverter())
 
     fun downloadsDirectory() = rxPrefs.getString(Keys.downloadsDirectory, defaultDownloadsDir.toString())