fix null pointer when infoWindow with altitude (#1428)

Author: fuzhenn

src/ui/InfoWindow.js

@@ -1,6 +1,7 @@
 import { isString } from '../core/util';
 import { createEl, addDomEvent, removeDomEvent } from '../core/util/dom';
 import Point from '../geo/Point';
+import Size from '../geo/Size';
 import { Geometry, Marker, MultiPoint, LineString, MultiLineString } from '../geometry';
 import UIComponent from './UIComponent';
 
@@ -30,6 +31,8 @@ const options = {
     'content': null
 };
 
+const EMPTY_SIZE = new Size(0, 0);
+
 /**
  * @classdesc
  * Class for info window, a popup on the map to display any useful infomation you wanted.
@@ -202,6 +205,9 @@ class InfoWindow extends UIComponent {
                 painter = children[0]._getPainter();
                 markerSize = children[0].getSize();
             }
+            if (!markerSize) {
+                markerSize = EMPTY_SIZE;
+            }
             if (painter) {
                 const fixExtent = painter.getFixedExtent();
                 o._add(fixExtent.xmax - markerSize.width / 2, fixExtent.ymin);