Description of problem:

Anaconda GUI installation crashes when ntp setting dialog is opened with incorrect IP and DNS server settings.

Version-Release number of selected component (if applicable):

CentOS Stream 8 

How reproducible:

Almost always (occurred 3 times in 4 attempts in my environment)

Steps to Reproduce:
1.  Boot with CentOS Stream 8 install ISO and launch anaconda GUI installer
2.  Click "System->Network & Host Name" button to open network settings pane
3.  Enable and set options of  the Ethernet  adapter which is used to communicate with ntp server as following
  a. IPv4 settings :  set to manual, configure a IP address and incorrect gateway address(not gateway)
  b. Additional DNS servers:  set DNS server address which cannot be reachable or incorrect(not DNS server) 
  c. (optional) disable IPv6 
4. Push "Done" button to exit Network settings
5.  Push "Localization-> Time & Date" button to open time and date settings pane
6.  Confirm the caution in the bottom of screen with the message "You have no working NTP server configured"
7.  Click  the "gear" icon button on the upper-right of screen to open NTP server configuration and wait for some seconds(about 10-20)
8. Confirm anaconda is crashed with the dialog "An unknown error has occurred

Actual results:

Anaconda installer is crashed.

Expected results:

Anaconda installer worked fine, ntp dialog can be modified.

Additional info:

I've attached two screenshots.

1. anaconda GUI screenshot with error
2. python stacktrace in /tmp/anaconda.log screenshot

When the dialog is opened, some multiple threads seems to be invoked which check if the ntp servers in the model(list) is working or not. The error seems to be occurred in handling the threads.