Make error message for adding multiple elements to a single-child panel more clear.
Make error message for adding multiple elements to a single-child panel more clear.

"This Panel" is pretty hard to decipher when parsing from XML.

blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon,blakeohare/crayon