When I open a form, it shows an error popup window: an unknown error has occurred during the execution of the modeler. It advises me to save changes and restart the modeler, however it is still the same when I restart the Modeler. Also when I try to create a new form, the same error window pops up, so I cannot continue with setting and making the form. In the error details it says "System.NullReferenceException: Object reference not set to an instance of an object."
The problem has been solved. Danchao Hu says:
"I've already solved the problem. The problem that I cannot open and editing forms seems due to my own computer problem, I've tried several laptop, because my computer is installed with Chinese version, when I try on other laptops with english version, the error never happen. Therefore I guess that might due to the reason of JDK language package."