Cannot focus programmatically, because inputRef becomes null after typing something into the MentionInput

[torlenor]

Steps to reproduce:

1. Open the demo locally or via https://react-mentions.vercel.app/
2. Go to Advanced options
3. Click somewhere so that the MentionsInput is not focused
4. Click on the button "focus programmatically"
5. It works
6. Now type something in the text field, click outside so that it is unfocused
7. Click on the button "focus programmatically"
8. It fails until you reload the page.

Expected behaviour:

Focus to work after you typed something in the text field

Observed behaviour:

Fails as soon as something was typed. The inputEl.current becomes null after you typed something.

Workaround:

Couldn't figure one out