fix: nil pointer when quitting simd start

[julienrbrt]

Description

Discovered while doing #13742:

• Exiting simd start gives a nil pointer
• Remove unused mock

---

Author Checklist

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• included the necessary unit and integration tests
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• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
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• reviewed state machine logic
• reviewed API design and naming
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[codecov]

Codecov Report

Merging #13743 (88b6bbf) into main (ad0e41d) will decrease coverage by 0.09%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13743      +/-   ##
==========================================
- Coverage   55.53%   55.43%   -0.10%     
==========================================
  Files         656      646      -10     
  Lines       56190    55779     -411     
==========================================
- Hits        31207    30923     -284     
+ Misses      22431    22340      -91     
+ Partials     2552     2516      -36     

Impacted Files	Coverage Δ
tx/textual/valuerenderer/string.go
tx/textual/valuerenderer/coins.go
tx/textual/valuerenderer/dec.go
tx/textual/valuerenderer/int.go
tx/textual/valuerenderer/bytes.go
tx/textual/valuerenderer/valuerenderer.go
tx/textual/valuerenderer/duration.go
tx/textual/valuerenderer/message.go
tx/textual/valuerenderer/timestamp.go
x/staking/simulation/operations.go	75.91% <0.00%> (+1.37%)	⬆️

[tac0turtle]

Lgtm. Nit: Could we fix the error check before merge