Upgrade Kotlin code for null safety

[MrCsabaToth]

Looks like the Kotlin code is not null safe in all places

[keysking]

Thanks for your attention. I will improve this work in the near future.

[MrCsabaToth]

I know it's an open source project, and I understand if you are busy, I wish I had more time to contribute.

[keysking]

Thank you very much for your understanding, I would be very grateful if I could receive your pr. Because that would be the first pr I received.
In addition, I have reinstalled the development environment to try to improve the whole plugin, such as using English comments and providing better examples.

[MrCsabaToth]

I won't have time to craft a PR in the near future unfortunately. I also had trouble with the IDE, that's the partial reason I opened the other issue errornously.