I know 2 reasons. First, this error usually occurs when you have specified a form name that does not exist in your <html: form> tag. For example, you specified <html: form action="/myAction" name="myForm" /> and 'myForm' is not the name of a form associated with myAction in the struts-config file
Secondly, in your form-bean you have some value object as an attribute with the setter and getter. One must initialize that value object at the time of loading of the form bean.