Initialize this.previousHash in the ThreadUI.init()

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Restore this.previousHash condition

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>
Restore ThreadUI.cleanFields param name and |this| alias

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>