Cause for this error message?

Question

Error message:
TypeError: Error #2007: Parameter child must be non-null.
at flash.display::DisplayObjectContainer/contains()
at main_fla::MainTimeline/releaseabout()
Script:
products.addEventListener(MouseEvent.CLICK, releaseabout);
function releaseabout(evt:MouseEvent):void {
if(contains(ldr)){
     removeChild(ldr);
if(contains(ldr1)){
     removeChild(ldr1);
if(contains(ldr2)){
     removeChild(ldr2);
if(contains(ldr3)){
     removeChild(ldr3);
if(contains(ldr4)){
     removeChild(ldr4);
if(contains(ldr5)){
     removeChild(ldr5);
if(contains(ldr6)){
     removeChild(ldr6);
gotoAndPlay(5);

Answer 1

This error can happen if you have declared an object ( var ldr:Loader; // has a null value ) but have no instance of it ( var ldr:Loader = new Loader(); )
If you expect to be in a situation such as this, you should probably first test for the null, then test for the presence...
if(ldr && contains(ldr)){
      removeChild(ldr);
I believe it will work as long as you order the tests this way.  Someone else may have a different/better solution.