Struts Wizard

First, sample application lacks other controls than text fields.
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So, when I use <html:checkbox property="test" /> control I get java.lang.NullPointerException in my Action class.
Why?

java.lang.NullPointerException
at net.aejase.plugin.wizard.WizardUtils.getActionFormFields(WizardUtils.java:120)
...

My struts-config.xml file is properly configured.
I've tried to do nothing in my Action class, but the java.lang.NullPointerException still occures.

My ActinForm contains properly defined methods:
public void setTest(boolean proba) { this.test=proba; }
public boolean isTest() { return test; }

Field test is initially set as false.
My ActinForm also contains adequate reset() method.