message: Use text question if there is an incorrect cellType in matrix dropdown.
diff: @@ -482,6 +482,9 @@ export class MatrixDropdownColumn extends Base implements ILocalizableOwner {
  }
  protected createNewQuestion(cellType: string): Question {
  var question = <Question>Serializer.createClass(cellType);
  + if (!question) {
  + question = <Question>Serializer.createClass("text");
  + }
  this.setQuestionProperties(question);
  return question;
  }
  