We have share How to empty TINYMCE content after AJAX action with jQuery. Use the mceRemoveControl execCommand here.
I do this:
tinyMCE.init({
selector: "textarea#mceEditorID",
});
if ( typeof tinyMCE != 'undefined') {
tinyMCE.EditorManager.execCommand('mceFocus', true, 'mceEditorID');
[...]