Fix NullPointerException on adding plugin to KMP project

[Virelion]

Seems like KMP project with targets other than JVM cause NullPointerException after connecting symbol processing plugin.
After that fix (and local publish) I was able to use JVM KSP in multiplatform project

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[Virelion]

@googlebot I signed it!

[ting-yuan]

Thanks for the PR!

[Virelion]

@ting-yuan Thank you for quick review. Is there any release planned anytime soon?

[ting-yuan]

Usually it's done bi-weekly, but this time we are going to have a hot fix today or tomorrow.

[ting-yuan]

The fix is included in the following release:
https://github.com/google/ksp/releases/tag/1.4.20-dev-experimental-20210111