Such an issue is likely to occur if there is a piece of code that tries to clear all the handlers attached to a DOM element, thus removing the form decorator event handlers, which will result in the mentioned JS error. Please, check if there is a place in your code where $clearHandlers is called.
Otherwise you should provide a sample runnable project
that demonstrates the issue at hand in order for me to reproduce it and investigate it in details. For that you should open a support ticket and attach the sample code there.
All the best,
the Telerik team
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