I recently faced an unusual issue with the default button to “insert an external link into Text field”. This is for Sitecore.NET 9.2.0 (rev. 002893). Every time I clicked on the web editor ribbon button, I got “Value cannot be null:html”. You are hence not able to proceed at all. Here I will show you an alternate approach to it. Sitecore has also raised a bug 371707 for this.
You can read my solution here.