Programmer - Calling function in other Class not working

Programmer Question

Hey everyone, not sure what is going on here :(

Basically I have a function that is needs to tell 2 other classes to do something. It works for one of the classes: BigPlayButton, but not Background for some reason.

TabMenu.as Class function

Note: The function below WILL call the hitPlayCircle function in my BigPlayButton class, but I get an undefined property error for the Background switchTitle function.

private function thumbClick(e:MouseEvent = null):void
    {
        trace("YOU CLICKED THUMBNAIL: " + e.target.id);
        trace("PLAY THIS VIDEO: " + tabData[tabID].video[e.target.id].@flv);
        trace("THE VIDEO TITLE: " + tabData[tabID].video[e.target.id].@title);

        newTitle = tabData[tabID].video[e.target.id].@title;
        Background.instance.switchTitle(newTitle);

        BigPlayButton.instance.playState = false;
        BigPlayButton.instance.hitPlayCircle(); // Hide the big play button

        vdp.setflvSource(tabData[tabID].video[e.target.id].@flv);
        vdp.playNewVideo(tabData[tabID].video[e.target.id].@flv);
    }

I've imported both classes so not sure what's going on :(
I did correctly set my static var instance variables.

public static var instance:Background; //<- in Background Class

public static var instance:BigPlayButton; // <- in BigPlayButton Class

And I have instance = this; in Both Classes as well...

The function inside my Background Class I'm trying to call from my TabMenu Class:

public function switchTitle(sentText):void
    {
        titleString = sentText;
        vTitle.text = titleString;
    }

Error Message (I always seem to get this error)

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at com.touchstorm.TEN.ui::TabMenu/thumbClick()

Find the answer here