stop() should be called only if start() called before

[Frosendroska]

Describe the bug
Every time when I am trying to stop the pipeline, I receive the following error:

`stop()` should be called only if `start()` called before

To Reproduce
Steps to reproduce the behavior:

1. Start the test case genertion.
2. Click on 'x' in the right bottom part of the screen to cancel the test case generation.
3. See error in the bottom right corner.
4. Click on the red !.

Expected behavior
Should stop the generation without an error.

Screenshots