Problem with access to instance methods from closures in SwingBuilder

An instance method is not found from inner node of SwingBuilder:

class StaticTest {
static void main(args)

public StaticTest () {
def swing = new groovy.swing.SwingBuilder()
def myFrame = swing.frame(size:[100,50]) {
panel() {
button(text: "test", actionPerformed:

)
}
}
myFrame.visible = true
}

private testMethod()

}

It gets visible if we:
1) remove panel(), and just place button in the form without panel
2) create a property in the class, assign 'this' to this property, and access the method via the property.