ReferenceError: Error #1056: Caused by Declaring Stage Instances Private

If you declare a stage instance private you get the message: "ReferenceError #1056 Cannot create property my_mc on StageIntanceDeclarationsClass"

This error occurs when you uncheck the "Declare Stage Instances Automatically" checkbox in the "ActionScript 3.0 Settings" dialogbox and proceed to declare stage instances as private variables in the class associated with the containing MovieClip.

A Note on Inheritances and Declaring Stage Instances:
You cannot choose to simply always declare stage instances automatically without forging the use of inheritance in classes linked to MovieClip Symbols. If you have a class APrime which is derived from class A and APrime is linked to a MovieClip Symbol, all stage instances used in the base class A must be manually declared in class A. "Declare Stage Instances Automatically" only declares instances in the class linked to the MovieClip Symbol and does NOT make those references available to any base classes.

Example:
Assume that the class StageIntanceDeclarations is set as the class associated with a MovieClip which c0ntains the MovieClip my_mc. Then the following code will cause ReferenceError #1056 at runtime.

package
{
        import flash.display.MovieClip;
        public class StageIntanceDeclarations extends MovieClip
        {
                //Private Causes ReferenceError #1056
                private var my_mc:MovieClip;
                function StageIntanceDeclarations()
                {
                }
        }
}

The output is as follows:
ReferenceError: Error #1056: Cannot create property my_mc on StageIntanceDeclarations.
at flash.display::Sprite/flash.display:Sprite::constructChildren()
at flash.display::Sprite$iinit()
at flash.display::MovieClip$iinit()
at TestStageIntanceDeclarationsBase$iinit()
at flash.display::Sprite/flash.display:Sprite::constructChildren()
at flash.display::Sprite$iinit()
at flash.display::MovieClip$iinit()

To avoid this error simply declare my_mc as public:
public var my_mc:MovieClip;