Replace use of check_plain() with HTML::escape()

Problem

check_plain() has been replaced with HTML::escape()

See change record: FormattableMarkup and HTML utility components replace check_plain(), format_string(), drupal_placeholder() (and SafeMarkup) →

Tasks

• make the change
• credit hendrik.fuss

Original Report (by hendrik.fuss)

Field formatter class does not work with the current release candidate 8.0.0-rc2. The module produces fatal php error messages and is thus unusable.

I am providing a patch here that seems to fix the issue.