Project Component wait for indexed directories

Hallo,
in my custom language plugin I want to attach a changeListener to three certain documents. I thought the best time to attach the listeners would be in a Project or Module Component.

I added the following code in the moduleAdded respective the projectOpened method of the components:

 final Document document = PsiDocumentManager.getInstance(project).getDocument(psiFile);

                document.addDocumentListener(new DocumentAdapter() {
                    @Override
                    public void documentChanged(DocumentEvent e) {
                        super.documentChanged(e);
                        System.out.println("I was changed");
                    }
                });

However, the PsiFile is null. I get the message : ERROR - .roots.impl.DirectoryIndexImpl - Directory index is not initialized yet for Project
So when is the right time to attach listeners to documents. Is there a action I can hook in that is called after all directories are indexed?
Thanks in advance,
Sebastian