Fix navigation error

[kendalsickels-wf]

@xvrh We've started to see an issue similar to #237 in our tests. I was able to create a test that fails consistently without any changes committed.

Then committed some changes that work around the issue but I'm lacking the context that you likely have over the entirety of this project to say if those changes would cause issues elsewhere.

I operated under the assumption that in the _onExecutionContextsCleared scenario we should go quietly into the night instead of erroring and _onExecutionContextsDestroyed we should error. But perhaps that was a poor assumption. Looking for any guidance/insight you might have on how to solve this issue.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (master@3fe1eeb). Click here to learn what that means.

Files	Patch %	Lines
lib/src/page/dom_world.dart	69.23%	4 Missing ⚠️
lib/src/page/frame_manager.dart	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #312   +/-   ##
=========================================
  Coverage          ?   88.80%           
=========================================
  Files             ?       32           
  Lines             ?     3172           
  Branches          ?        0           
=========================================
  Hits              ?     2817           
  Misses            ?      355           
  Partials          ?        0           

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[xvrh]

Thanks for the contribution. I'll merge it as it: all the tests are passing and it seems to fix already some other issues (like #314)