Hi Cecil,
This is the first time we receive such error report for the AppBuilder Simulator. Performing a search about the exact exception message, I found the following
article. It makes me think that if you clear the Simulator's cache, the faulty behavior should correct itself. Could you please close any opened VS instances and then, delete the whole
%localappdata%\Telerik\BlackDragon\Cache folder on your machine and try again?
Please, let me know if the issue is still present after that.
Regards,
Kaloyan
Telerik