ActionScript 3.0 :: Declare Variable For Symbol Instance

I want to declare a variable in a class for an instance of an object that was added in the Flash IDE.

Example:
I have symbols for several types of panels that all subclass Panel.
To each panel symbol in the library, I add a button and give it an instance name "defaultButton."

This is really convenient, however, I want a defaultButton property for for my Panel class, and cannot find a way to do it. If I try adding a defaultButton property it throws errors:

ActionScript Code:
public class Panel: extends MovieClip{
public var defaultButton:Button; //as an example. Getters and setters also don't work
}

I also cannot check for the existence of defaultButton in code:
ActionScript Code:
public function GetDefaultButton():Button {
if(this.defaultButton)
return this.defaultButton;
else

Throw new Error("Panels are supposed to have a Button instance named 'defaultButton' added using the Flash IDE");
}
...possibly undefined error.

Is there any way for my class to enforce/ recognise the existence of the defaultButton instance added using the IDE in my Class?