[Test Step Field] "Invalid user" message is shown when instead of the user name that was selected for the user picker type of the test step field

Steps to reproduce:

1. Add an "User Picker" type test step field
2. Create the manual test
3. Select some user for the field from step 1

Actual result: "Invalid user" message is shown.
Expected result: Selected user is shown in the field with no errors.

Note: after reloading the page, the right user is shown in the field