[FIX]ObservableToPropertyHelper thread problems; fixes #1298

[lxatstariongroup]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following the COMET-IME code style guidelines
• [x] I have provided test coverage for my change (where applicable)

Description

[FIX]ObservableToPropertyHelper thread problems; fixes #1298