Add isLoaded check to RichText#getValue to avoid errors when the iframe has not yet loaded.
Fixes #15134.
(cherry picked from commit 4dc879a433b3c3db4310011cf28e13edd0801139)

(cherry picked from commit 73c826455d90aefa088a165b6fc240edbcf80199)

Conflicts:
	_editor/RichText.js
	tests/editor/Editor.html