这个方法将geojson字符串转为Geometry对象的时候一直返回null
传入的geojson如下：
{"points":[{"x":18375717.577550508,"y":3389496.0663330085},{"x":18379851.83976489,"y":3387378.5191295836},{"x":18379549.333571054,"y":3377496.623846022},{"x":18371936.240511123,"y":3383496.346051159},{"x":18375717.577550508,"y":3389496.0663330085}]}
