Potential null value of `WidgetsBinding.instance`

[guillempuche]

It could not access toFocusManager because the instance of WidgetsBinding could be null.

Environment:

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List of issues which are fixed by the PR

Error:

Property `focusManager` cannot be accessed on `WidgetBinding` because it is potentially null.

Screenshots

Checklist

• [X] I signed the [CLA].
• [X] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making].
• [ ] All existing and new tests are passing.

[jenshor]

This problem is quite tricky since Flutter 3.0 changed the instance property from nullable to non-nullable. Therefore, your change will cause problems for Flutter 3.0.

[guillempuche]

Then, could you change the CHANGELOG to reflect when this was changed and explain the requirements (we need Flutter 3) on the main documentation?

[yousinix]

Widgetbook next stable release will not support Flutter 2.0.