message: Fix error in matrix dropdown if all columns are invisible.
diff: @@ -1100,7 +1100,7 @@ export class QuestionMatrixDropdownModelBase
  for (var i = 0; i < this.generatedVisibleRows.length; i++) {
  var cells = this.generatedVisibleRows[i].cells;
  if (!cells) continue;
  - for (var colIndex = 0; colIndex < this.columns.length; colIndex++) {
  + for (var colIndex = 0; colIndex < cells.length; colIndex++) {
  if (!cells[colIndex]) continue;
  var question = cells[colIndex].question;
  res =
  @@ -1122,7 +1122,7 @@ export class QuestionMatrixDropdownModelBase
  if (!this.generatedVisibleRows) return null;
  for (var i = 0; i < this.generatedVisibleRows.length; i++) {
  var cells = this.generatedVisibleRows[i].cells;
  - for (var colIndex = 0; colIndex < this.columns.length; colIndex++) {
  + for (var colIndex = 0; colIndex < cells.length; colIndex++) {
  if (!onError) return cells[colIndex].question;
  if (cells[colIndex].question.currentErrorCount > 0)
  return cells[colIndex].question;
  