Calling dismissBulletin whilst bulletin is dismissing causes crash

If I call `bulletinManager.dismissBulletin(animated: true)` when the bulletin has already been dismissed, it crashes saying `BLTNItemManager must only be used from the main thread.` If the `dismissBulletin()` function is called immediately after the manager has been dismissed, `bulletinManager.isShowingBulletin` is still `true`. There needs to be a check like "if the manager is no longer presenting, or is in the middle of dismissing, ignore the call to dismiss".

I'm using the latest version of BulletinBoard