I'm working on a recursive method that displays all documents I have permissions to see.  The first pass works great, but when it calls itself recursively passing a document array of the current document's children it throws an error:

Object reference not set to an instance of an object.

Here's the code:

protected void Page_Load(object sender, EventArgs e)
        {
            lblTest.Text = "Data<br /><br />";
            Document[] releaseDocs = Document.GetRootDocuments();
            displayDocs(releaseDocs);
        }
public void displayDocs(Document[] releaseDocs)
        {
            string docPermissions = null;
            User currentUser = User.GetCurrent();
            foreach (var doc in releaseDocs)
            {
                docPermissions = currentUser.GetPermissions(doc.Path);
                if ((docPermissions.Contains("F")) && (docPermissions.Contains("U")))
                {
                    lblTest.Text += "D/T: " + doc.CreateDateTime + "<br />\r\n";
                    lblTest.Text += "Level: " + doc.Level + "<br />\r\n";
                    lblTest.Text += "Text: " + doc.Text + "<br />\r\n";
                    lblTest.Text += "<hr />\r\n";
                    if (doc.HasChildren)
                    {
                        Document[] childDocs = Document.GetChildrenForTree(doc.Id);
                        displayDocs(childDocs); //error occurs here
                    }
                }
            }
        }