anchor.diff:        environment.whereType<_UndefinedExpression>();
   
     void _recursiveGetExpressions(
  -      List<Expression?> expressions, Iterable<Element?> elements, bool? value) {
  +      List<Expression> expressions, Iterable<Element?> elements, bool? value) {
       for (var element in environment.reversed) {
         if (element.haveToStop(elements)) {
           return;
         }
         if (element is _ConditionExpression && element.value == value) {
  -        expressions.add(element.expression);
  +        var expression = element.expression;
  +        if (expression != null) {
  +          expressions.add(expression);
  +        }
         }
       }
       outer?._recursiveGetExpressions(expressions, elements, value);
  
positive.code: }
  
  class _ConditionExpression extends ExpressionBox {
    Expression? expression;
    bool value;
  
    _ConditionExpression(this.expression, {this.value = true});
  
  
negative.code: }
  
  class _UndefinedExpression extends ExpressionBox {
    final Element element;
  
    _UndefinedExpression._internal(this.element);
  
  