For the most part it looks fine. There are a few parts that may be changed. For example, I think that the handle handleDropboxAction method and the usage of the DropboxActionResolver functional interface might be a bit of an overkill.
You could achieve the exact same behavior without creating a custom interface to essentially wrap it an try-catch clause.
listFolder arguments could be made primitive (to avoid the null checks).
Your various throws DropboxException declaration could be omitted.