老师，程序概率性（概率为1/30）出现如下错误，怎么提高代码的健壮性？

来源：5-14 多进程、unittest、HTMLTestRunner、启动服务流程梳理代码重构

慕莱坞6458913

2018-11-01

Traceback (most recent call last):
File “D:\Users\uidq1501\eclipse-workspace\android_test\src\case\factory_reset.py”, line 87, in
Factory_reset(0).test_case()
File “D:\Users\uidq1501\eclipse-workspace\android_test\src\case\factory_reset.py”, line 32, in test_case
SourceBusiness(0).factory_reset()
File “D:\Users\uidq1501\eclipse-workspace\android_test\src\business\source_switch_business.py”, line 13, in init
self.handle = SourceHandle(i)
File “D:\Users\uidq1501\eclipse-workspace\android_test\src\handle\source_switch_handle.py”, line 11, in init
self.page = Source_Page(i)
File “D:\Users\uidq1501\eclipse-workspace\android_test\src\page\source_switch_page.py”, line 23, in init
self.driver = base_driver.desired_setting(i)
File “D:\Users\uidq1501\eclipse-workspace\android_test\src\util\desired_setting.py”, line 30, in desired_setting
driver = webdriver.Remote(“http://127.0.0.1:”+port+"/wd/hub",desired_caps)
TypeError: cannot concatenate ‘str’ and ‘NoneType’ objects