VNADatabaseWillDeleteFolderNotification race condition

[Eitot]

I came across this while working on #1986.

vienna-rss/Vienna/Sources/Database/Database.m

Lines 1078 to 1087 in 41254da

	// Send the pre-delete notification before we start the transaction so that the handlers can
	// safely do any database access.
	for (folder in arrayOfChildFolders) {
	numFolder = @(folder.itemId);
	[arrayOfFolderIds addObject:numFolder];
	[[NSNotificationCenter defaultCenter] vna_postNotificationOnMainThreadWithName:VNADatabaseWillDeleteFolderNotification object:numFolder];
	}
	// Now do the deletion.
	result = [self wrappedDeleteFolder:folderId];

wrappedDeleteFolder: is sent after the notification is posted, but before any notification observers are notified. ActivityLog and RefreshManager use this notification to retrieve the folder from the database using the provided folder identifier, but by that time the folder has been deleted already.