Fix potential twice theme route handler invocations

[JohnNiang]

What type of PR is this?

/kind bug
/area core
/milestone 2.20.x

What this PR does / why we need it:

1. This PR removes duplicate invocations while resolving handler functions of theme.
2. Throw NotFoundException while post was not found.

Which issue(s) this PR fixes:

Fixes #7409

Does this PR introduce a user-facing change?

修复访问不存在的分类或者文章页面时始终抛出异常的问题

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.44%. Comparing base (60c2009) to head (d7c7b78).
Report is 33 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7419      +/-   ##
============================================
+ Coverage     57.18%   59.44%   +2.26%     
- Complexity     4066     4114      +48     
============================================
  Files           721      697      -24     
  Lines         24546    23948     -598     
  Branches       1612     1548      -64     
============================================
+ Hits          14036    14236     +200     
+ Misses         9882     9059     -823     
- Partials        628      653      +25     

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[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ruibaby

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