fix: clicking on back button after post creation

[DmytroAlipov]

Description

Steps to Reproduce:

1. Open course in LMS

2. Open discussions sidebar

3. Add post -> Submit
   

4. Click on back browser button
   

5. Result
   

Result of the investigation:

Found that post id is unpacked when saving to state, although it should be saved intact:

After fixing the unpacking, this error also started to occur:

I have added a check that the post exists to fix this situation:

[openedx-webhooks]

Thanks for the pull request, @DmytroAlipov!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.18%. Comparing base (3cc39d8) to head (589c862).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #745   +/-   ##
=======================================
  Coverage   93.17%   93.18%           
=======================================
  Files         161      161           
  Lines        3401     3402    +1     
  Branches      919      920    +1     
=======================================
+ Hits         3169     3170    +1     
+ Misses        215      214    -1     
- Partials       17       18    +1     

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[mphilbrick211]

Hi @openedx/2u-infinity! This is ready for review.