Fix the timing of setting the editor value

[sergeystoma]

Related to https://github.com/fagbokforlaget/strapi-plugin-wysiwyg-toastui/issues/3

[iapain]

@shardiwal Could you please verify this PR?

[JoshuaJWayne]

@sergeystoma Thank you for this patch it fixed the issue I was having perfectly.

[wrrwrr111]

can use componentDidUpdate

componentDidUpdate(prevProps, prevState, snapshot) {
    if (this.props.value !== prevProps.value && prevProps.value == null) {
      const editor = this.editorRef.current.getInstance();
      editor.setMarkdown(this.props.value, false);
    }
}