Unfortunately, the error does not say which element is in error. Could do with a more specific error here. When you get such a generic error it usually means there is a syntax error in the XML file. Tools like Visual Studio Code usually help you with that.
Could be as simple as a missing close tag. Also, each property must have a Description tag but you may leave the value empty.
Double check the XML file for syntax errors. I have had a very persistent one with a new widget I wanted to create. I ended up removing all properties except a simple one and adding them back in until I found the error. Also turned out to be a syntax error.