fix: bind _valueChanged method in editor constructor

Fixed "Cannot read properties of undefined (reading 'bind')" error
in visual editor by properly binding the _valueChanged method to
the class instance in the constructor.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: nurbiy.teuchezh