Small fix

app/src/main/java/eu/faircode/email/WorkerCleanup.java

@@ -217,6 +217,7 @@ public class WorkerCleanup extends Worker {
             Log.breadcrumb("worker", "cleanup", "message files");
             {
                 File[] files = new File(context.getFilesDir(), "messages").listFiles();
+                if (files != null)
                     for (File file : files) {
                         if (file.isDirectory())
                             cleanupMessageFiles(db, manual, file.listFiles());