Sometimes when starting a test using Pick a browser to open a new Client Test Runner, the client test runner reports an error "step_execution_generator' of undefined" and does not run the test. Running the test again will succeed.
Steps to Reproduce
Create an ATF test with a UI-based step.
Close all Client Test Runner windows.
Run the test.
The test fails with error "Step execution failed with error: Cannot read property 'step_execution_generator' of undefined". Running the test again will succeed.
Open the Client Test Runner (available as a module in Automated Test Framework application menu) before running a test.
Related Problem: PRB1243954