message: assign default height after assigning new sections
  also apply it to sections with value null
diff: @@ -63,9 +63,10 @@ export class Layout {
  if (Number.isFinite(availableHeight)) {
  this._availableHeight = availableHeight;
  }
  + this._sections = sections;
  const totalHeight = this._getAvailableHeight();
  this._sections.forEach((section, i) => {
  - if (!this._sectionHeights.hasOwnProperty(section.id)) {
  + if (!this._sectionHeights[section.id]) {
  this._sectionHeights[section.id] = clamp(
  totalHeight / this._sections.length,
  this._getMinHeight(i),
  @@ -73,7 +74,6 @@ export class Layout {
  );
  }
  });
  - this._sections = sections;
  this._applyNewSize();
  }
  