问AS3错误#2025年:提供的DisplayObject必须是调用方的子级。当试图带走一个孩子时

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stage.addEventListener(MouseEvent.MOUSE_UP, mUp);
function mUp(MouseEvent): void {

var textfield = new TextField();
textfield.type = TextFieldType.INPUT
textfield.x = mouseX;
textfield.y = mouseY;
stage.focus = textfield;
textfield.selectable = false;
stage.addChild(textfield);  // adding the child here 
}

function erase(evt: MouseEvent): void {   //triggers when button is clicked

stage.removeChild(textfield)  //trying to remove the child, but throws the error
}

var board: Sprite = new Sprite();  // displayobjectcontainer to hold the textfields
addChild(board);
var textfield:TextField;  // Declared outside the functions

listener.addEventListener(MouseEvent.MOUSE_UP, mUp);  // I added an object on the stage to catch my mouse input instead of the stage, so that it doesn't trigger when I click my button

function mUp(evt:MouseEvent):void 
{
textfield = new TextField();  // I have this still so it will create a new textfield on every mUP
textfield.type = TextFieldType.INPUT
textfield.x = mouseX;
textfield.y = mouseY;
stage.focus = textfield;
textfield.selectable = false;
board.addChild(textfield);  // adding the child to the sprite now 
}

function erase(evt:MouseEvent):void
{
board.removeChild(textfield)  //trying to remove the child, but still throws                the error
}