During creation of a new test case, if we select default tester first then if we select any component, the selected default tester field is going to be empty. It looks mutually exclusive. BUT, if we select first component, then Default Tester, is working properly. In real time, when a user create a test case, he has to select first default tester thenafter component.
This issue is not fixable by us at the moment as it is Testopia functionality related. The Mozilla Foundation is developing Testopia so please use https://bugzilla.mozilla.org/ to address this issue.
Setting to verified.