question: @ Override public void addChild ( FXGNode child ) { if ( child instanceof GraphicContentNode ) { if ( child instanceof TextGraphicNode ) textCount ++ ; if ( children == null ) children = new ArrayList < GraphicContentNode > ( ) ; children . add ( ( GraphicContentNode ) child ) ; if ( child instanceof GroupNode ) { if ( definesScaleGrid ( ) ) { throw new FXGException ( child . getStartLine ( ) , child . getStartColumn ( ) , "InvalidScaleGridGroupChild" ) ; } } } else { super . addChild ( child ) ; } }
target:  Add a child node. An exception is thrown if the child node is not   a GraphicContentNode or a child is a group and the current group  has scale grid defined.