message: Fix undefined map when switching to offscreen viewport.
diff: @@ -546,7 +546,8 @@ export class ViewFrustum {
  }
  private calcNpcToView(): Map4d {
  const corners = this.getViewCorners();
  - return Map4d.createBoxMap(NpcCorners[Npc._000], NpcCorners[Npc._111], corners.low, corners.high)!;
  + const map = Map4d.createBoxMap(NpcCorners[Npc._000], NpcCorners[Npc._111], corners.low, corners.high);
  + return undefined === map ? Map4d.createIdentity() : map;
  }
  /* Get the extents of this view, in ViewCoordinates, as a Range3d */
  