Fixed editor throwing an exception when destroyed shortly after it was created

[mlewand]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches which change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

Skipped the manual test as this is pretty much an internal change, and the case presented for manually reproduced the issue required refreshing the page.

What changes did you make?

This PR adds two early returns, making sure that delayed editor initialization doesn't take place in case it was removed before.

4 people in total worked on this one :D I had to adjust the unit tests, as they were not failing for previous CKEditor releases, thus it would not protect us from a regression.

This is a follow-up of a #273 PR.

Closes #2257, #718.

[msamsel]

Last commit (fix for IE8) makes test always pass.

[jacekbogdanski]

I wonder if we shouldn't add a manual test for this changes. As I understand this issue somehow touched users using CKEditor so IMO we should cover it with a manual test, especially that current unit test is false positive and may give invalid result in the future.

[jacekbogdanski]

The test always passes despite changes.

[mlewand]

Fixed the unit test.

I decided to remove wysiwygarea change, as it does not fix the issue, there's still an option to break it with the following code: https://gist.github.com/mlewand/ee09f6dc0148792c8d08f53cc55da605 However fixing it would take more time (it would need to be done very carefully in core setMode logic) and it's just an edge case.

As for manual test I related to that in:

Skipped the manual test as this is pretty much an internal change, and the case presented for manually reproduced the issue required refreshing the page.

[jacekbogdanski]

The unit test is still very unstable on IE8. It fails half of a time.

[f1ames]

Closing this PR as there is already more general PR on the way #3128.