The provided code seems correct. The only issue that I found, which is not related to the template error is that the two widgets TabStrip and NumericTextBox will have identical names, so please rename one of them.
I am assuming that the error is caused by the AntiXss encoder. Please remove the following line from your Web.config file if it is present.
"System.Web.Security.AntiXss.AntiXssEncoder, System.Web, Version=22.214.171.124, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
I hope that this information was helpful for you. I wish you a great day!
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