Fix NPE when calling with bib file as cmd argument (#3343)

Use toAbsolutePath before calling of getParent

src/main/java/org/jabref/gui/util/FileUpdateMonitor.java

@@ -71,7 +71,7 @@ private void notifyAboutChange(Path path) {
  */
  public void addListenerForFile(Path file, FileUpdateListener listener) throws IOException {
  // We can't watch files directly, so monitor their parent directory for updates
- Path directory = file.getParent();
+ Path directory = file.toAbsolutePath().getParent();
  directory.register(watcher, StandardWatchEventKinds.ENTRY_MODIFY);
 
  listeners.put(file, listener);