Removes unnecessary nullability check warnings after Flutter 3.0.x upgrade.
Checklist
Remove If [...] items that do not apply to your PR.
[x] I added a PR description.
[x] If this PR changes anything about the main math_keyboard or example package
(also README etc.), I created an entry in CHANGELOG.md (## UPCOMING RELEASE if the change
on its own is not worth an update).
[x] If this PR includes a notable change in the math_keyboard package, I updated the version
according to Dart's semantic versioning.
Description
Removes unnecessary nullability check warnings after Flutter 3.0.x upgrade.
Checklist
Remove
If [...]items that do not apply to your PR.math_keyboardorexamplepackage (also README etc.), I created an entry inCHANGELOG.md(## UPCOMING RELEASEif the change on its own is not worth an update).math_keyboardpackage, I updated the version according to Dart's semantic versioning.