This PR addresses around 500 TypeScript errors by enabling
strictNullChecks in the richtext-lexical package. In the process,
several bugs were identified and fixed.
In some cases, I applied non-null assertions where necessary, although
there may be room for further type refinement in the future. The focus
of this PR is to resolve the immediate issues without introducing
additional technical debt, rather than aiming for perfect type
definitions at this stage.
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Co-authored-by: Alessio Gravili <alessio@gravili.de>
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