message: allow to set to matrix.cells a null value (clear cells):
diff: @@ -131,9 +131,11 @@ export class MartrixCells {
  this.values = {};
  if (!value) return;
  for (var row in value) {
  + if (row == "pos") continue;
  var rowValues = value[row];
  this.values[row] = {};
  for (var col in rowValues) {
  + if (col == "pos") continue;
  var loc = new LocalizableString(this.cellsOwner);
  loc.setJson(rowValues[col]);
  this.values[row][col] = loc;
  @@ -230,8 +232,7 @@ export class QuestionMatrixModel extends Question
  return this.cellsValue;
  }
  public set cells(value: MartrixCells) {
  - if (!value) return;
  - this.cells.setJson(value.getJson());
  + this.cells.setJson(value && value.getJson ? value.getJson() : null);
  }
  public get hasCellText(): boolean {
  return !this.cells.isEmpty;
  