Fix caretRangeFromPoint for Firefox

[ellatrix]

Description

Currently there is an error in Firefox when moving the caret down with the arrow key, and the target block is outside of the viewport. In that case caretPositionFromPoint returns null and range.setStart fails. There will be no scrolling with a second attempt. To fix this we have to return null to prevent the error, and a second attempt will be made by scrolling the page.

How has this been tested?

See above.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[aduth]

Confirmed resolves error 👍

Is there some documentation / specification that describes this behavior we could include as a reference link? Or mention that it's Firefox-specific?