NPE com.sun.mail.iap.Response com.sun.mail.imap.protocol.IMAPProtocol.readIdleResponse()

**Describe the bug**

Occasionally I see this in [Bugsnag](https://www.bugsnag.com/) for [FairEmail](https://github.com/M66B/FairEmail/):

```
java.lang.NullPointerException: Attempt to invoke virtual method 'com.sun.mail.iap.Response com.sun.mail.imap.protocol.IMAPProtocol.readIdleResponse()' on a null object reference
at com.sun.mail.imap.IMAPFolder.handleIdle(SourceFile:3227)
at com.sun.mail.imap.IMAPFolder.idle(SourceFile:3135)
```

JavaMail version 1.6.4/Android

The reported entry points are com.sun.mail.imap.IMAPFolder.handleIdle and idle, where com.sun.mail.imap.protocol.IMAPProtocol.readIdleResponse is called on a null object. Start by tracing how the protocol object reaches handleIdle in JavaMail 1.6.4 on Android; done means identifying the null path and resolving the reported NPE.

Written by the indexing model from the issue text.