This may be a bug. Open diagram designer, draw a line or line connector then double click on it. You will see the place holder when you can enter some text. Don't add any text, just click on an empty space. Now double click again, you can see the place holder to enter the text never comeback for this arrow or line.

just try that and let me know

Hi,

thank you for reporting this bug. It is users like you that help us make our components even better. We'll investigate the problem and fix it as soon as possible.

Best Regards,
Nevron Support Team