Error on activation & creating form, widget doesn’t appear

On activation:
The plugin generated 1413 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

When trying to create a form in WordPress:

Notice: Undefined index: form_type in /home/customer/www/quotesforclarity.com/public_html/wp-content/plugins/official-mailerlite-sign-up-forms/include/mailerlite-admin.php on line 582

Warning: Cannot modify header information - headers already sent by (output started at /home/customer/www/quotesforclarity.com/public_html/wp-content/plugins/official-mailerlite-sign-up-forms/include/mailerlite-admin.php:582) in /home/customer/www/quotesforclarity.com/public_html/wp-includes/pluggable.php on line 1296

The form did get created despite the error message, and I was able to go back in and edit it. It worked for a little while, but then I edited it again and it stopped appearing anymore in the sidebar — the widget is just not shown.

I was able to use the form by adding a Text widget and using the shortcode.

This plugin does not seem production-ready.